| Message ID | 1346021216-21979-3-git-send-email-ccross@android.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Hi, On 8/26/2012 3:46 PM, Colin Cross wrote: > Unwinding with CONFIG_ARM_UNWIND is much more complicated than > unwinding with CONFIG_FRAME_POINTER, but there are only a few points > that require validation in order to avoid faults or infinite loops. > Avoiding faults is easy by adding checks to verify that all accesses > relative to the frame's stack pointer remain inside the stack. > > When CONFIG_FRAME_POINTER is not set it is possible for two frames to > have the same SP, so there is no way to avoid repeated calls to > unwind_frame continuing forever. > > Signed-off-by: Colin Cross <ccross@android.com> > --- This is a feature we've wanted for a long time. I ran some test with repeated catting of stacks with processes dying and the results looked good. You can add a Tested-by: Laura Abbott <lauraa@codeaurora.org> Laura
On Sun, Aug 26, 2012 at 03:46:56PM -0700, Colin Cross wrote: > Unwinding with CONFIG_ARM_UNWIND is much more complicated than > unwinding with CONFIG_FRAME_POINTER, but there are only a few points > that require validation in order to avoid faults or infinite loops. > Avoiding faults is easy by adding checks to verify that all accesses > relative to the frame's stack pointer remain inside the stack. > > When CONFIG_FRAME_POINTER is not set it is possible for two frames to > have the same SP, so there is no way to avoid repeated calls to > unwind_frame continuing forever. So here you admit that this patch can cause the unwinder to loop forever, which would provide no way out of that. Why do you think this patch is suitable for mainline with such a problem?
On Fri, Oct 12, 2012 at 10:08:07AM +0100, Russell King - ARM Linux wrote: > On Sun, Aug 26, 2012 at 03:46:56PM -0700, Colin Cross wrote: > > Unwinding with CONFIG_ARM_UNWIND is much more complicated than > > unwinding with CONFIG_FRAME_POINTER, but there are only a few points > > that require validation in order to avoid faults or infinite loops. > > Avoiding faults is easy by adding checks to verify that all accesses > > relative to the frame's stack pointer remain inside the stack. > > > > When CONFIG_FRAME_POINTER is not set it is possible for two frames to > > have the same SP, so there is no way to avoid repeated calls to > > unwind_frame continuing forever. > > So here you admit that this patch can cause the unwinder to loop forever, > which would provide no way out of that. Why do you think this patch is > suitable for mainline with such a problem? With CONFIG_FRAME_POINTER we have a straightforward definition of progress: the sp must increase per frame, and cannot increase beyond the limit of the tasks stack. We get this property from the fact that each frame record consumes actual space on the stack. If we parse a frame record which does not both increase the sp and provide a frame address greater than that sp, we know that frame is garbage and we must stop. With CONFIG_ARM_UNWIND, we have no straightforward definition of progress. However, sp must _normally_ still increase, because compiler- generated non-leaf functions must store the lr somewhere, and the compiler always uses the stack. Even if lr is stashed in r4, an ABI compliant would then have needed to save r4 on the stack beforehand. We can assume that we will never parse a garbage unwind table because of the way the table lookup works (though we may parse a valid table which has nothing whatever to do with the code that was executing in the case of a corrupted stack). So we only need to worry about what the unwind tables will look like for valid functions. Nonetheless, tail-call-optimised and manually-annotated functions may have unusual frames which don't consume any stack. Stackless tail- call-optimised functions shouldn't be a problem, since their frames are completely missing from the backtrace and won't dump us into a loop. Stackless assembler functions are overwhelmingly likely to be leaf functions, giving us just one stackless frame. Would it make sense if we place some small sanity limit on the number of frames the unwinder will process with the same sp before giving up? Cheers ---Dave
On Fri, Oct 12, 2012 at 3:02 AM, Dave Martin <dave.martin@linaro.org> wrote: > On Fri, Oct 12, 2012 at 10:08:07AM +0100, Russell King - ARM Linux wrote: >> On Sun, Aug 26, 2012 at 03:46:56PM -0700, Colin Cross wrote: >> > Unwinding with CONFIG_ARM_UNWIND is much more complicated than >> > unwinding with CONFIG_FRAME_POINTER, but there are only a few points >> > that require validation in order to avoid faults or infinite loops. >> > Avoiding faults is easy by adding checks to verify that all accesses >> > relative to the frame's stack pointer remain inside the stack. >> > >> > When CONFIG_FRAME_POINTER is not set it is possible for two frames to >> > have the same SP, so there is no way to avoid repeated calls to >> > unwind_frame continuing forever. >> >> So here you admit that this patch can cause the unwinder to loop forever, >> which would provide no way out of that. Why do you think this patch is >> suitable for mainline with such a problem? > > With CONFIG_FRAME_POINTER we have a straightforward definition of > progress: the sp must increase per frame, and cannot increase beyond the > limit of the tasks stack. We get this property from the fact that > each frame record consumes actual space on the stack. If we parse a > frame record which does not both increase the sp and provide a frame > address greater than that sp, we know that frame is garbage and we must > stop. > > > With CONFIG_ARM_UNWIND, we have no straightforward definition of > progress. However, sp must _normally_ still increase, because compiler- > generated non-leaf functions must store the lr somewhere, and the > compiler always uses the stack. Even if lr is stashed in r4, an ABI > compliant would then have needed to save r4 on the stack beforehand. > > We can assume that we will never parse a garbage unwind table because of > the way the table lookup works (though we may parse a valid table which > has nothing whatever to do with the code that was executing in the case > of a corrupted stack). So we only need to worry about what the unwind > tables will look like for valid functions. > > Nonetheless, tail-call-optimised and manually-annotated functions may > have unusual frames which don't consume any stack. Stackless tail- > call-optimised functions shouldn't be a problem, since their frames > are completely missing from the backtrace and won't dump us into a loop. > Stackless assembler functions are overwhelmingly likely to be leaf > functions, giving us just one stackless frame. > > > Would it make sense if we place some small sanity limit on the number > of frames the unwinder will process with the same sp before giving up? About half the callers to unwind_frame end up limiting the number of frames they will follow before giving up, so I wasn't sure if I should put an arbitrary limit in unwind_frame or just make sure all callers are bounded. Your idea of limiting same sp frames instead of total frames sounds better. I can send a new patch that adds a new field to struct stackframe (which will need to be initialized everywhere, the struct is usually on the stack) and limits the recursion. Any suggestion on the recursion limit? I would never expect to see a real situation with more than a few, but on the other hand parsing the frames should be pretty fast so a high number (100?) shouldn't cause any user visible effect.
On Mon, Oct 15, 2012 at 07:15:31PM -0700, Colin Cross wrote: > On Fri, Oct 12, 2012 at 3:02 AM, Dave Martin <dave.martin@linaro.org> wrote: > > On Fri, Oct 12, 2012 at 10:08:07AM +0100, Russell King - ARM Linux wrote: > >> On Sun, Aug 26, 2012 at 03:46:56PM -0700, Colin Cross wrote: > >> > Unwinding with CONFIG_ARM_UNWIND is much more complicated than > >> > unwinding with CONFIG_FRAME_POINTER, but there are only a few points > >> > that require validation in order to avoid faults or infinite loops. > >> > Avoiding faults is easy by adding checks to verify that all accesses > >> > relative to the frame's stack pointer remain inside the stack. > >> > > >> > When CONFIG_FRAME_POINTER is not set it is possible for two frames to > >> > have the same SP, so there is no way to avoid repeated calls to > >> > unwind_frame continuing forever. > >> > >> So here you admit that this patch can cause the unwinder to loop forever, > >> which would provide no way out of that. Why do you think this patch is > >> suitable for mainline with such a problem? > > > > With CONFIG_FRAME_POINTER we have a straightforward definition of > > progress: the sp must increase per frame, and cannot increase beyond the > > limit of the tasks stack. We get this property from the fact that > > each frame record consumes actual space on the stack. If we parse a > > frame record which does not both increase the sp and provide a frame > > address greater than that sp, we know that frame is garbage and we must > > stop. > > > > > > With CONFIG_ARM_UNWIND, we have no straightforward definition of > > progress. However, sp must _normally_ still increase, because compiler- > > generated non-leaf functions must store the lr somewhere, and the > > compiler always uses the stack. Even if lr is stashed in r4, an ABI > > compliant would then have needed to save r4 on the stack beforehand. > > > > We can assume that we will never parse a garbage unwind table because of > > the way the table lookup works (though we may parse a valid table which > > has nothing whatever to do with the code that was executing in the case > > of a corrupted stack). So we only need to worry about what the unwind > > tables will look like for valid functions. > > > > Nonetheless, tail-call-optimised and manually-annotated functions may > > have unusual frames which don't consume any stack. Stackless tail- > > call-optimised functions shouldn't be a problem, since their frames > > are completely missing from the backtrace and won't dump us into a loop. > > Stackless assembler functions are overwhelmingly likely to be leaf > > functions, giving us just one stackless frame. > > > > > > Would it make sense if we place some small sanity limit on the number > > of frames the unwinder will process with the same sp before giving up? > > About half the callers to unwind_frame end up limiting the number of > frames they will follow before giving up, so I wasn't sure if I should > put an arbitrary limit in unwind_frame or just make sure all callers > are bounded. Your idea of limiting same sp frames instead of total > frames sounds better. I can send a new patch that adds a new field to > struct stackframe (which will need to be initialized everywhere, the > struct is usually on the stack) and limits the recursion. Any > suggestion on the recursion limit? I would never expect to see a real > situation with more than a few, but on the other hand parsing the > frames should be pretty fast so a high number (100?) shouldn't cause > any user visible effect. Talking to some tools guys about this, it sounds like there really shouldn't be any stackless frame except for the leaf frame. If there are stackless functions they will probably not be visible in the frame chain at all. So it might make sense to have a pretty small limit. Maybe it could even be 1. Cartainly a small number. We should also add a check for whether the current and frame and previous frame appear identical and abort if that's the case, if we don't do that already. Cheers ---Dave
On Tue, Oct 16, 2012 at 11:12:01AM +0100, Dave Martin wrote: > On Mon, Oct 15, 2012 at 07:15:31PM -0700, Colin Cross wrote: > > About half the callers to unwind_frame end up limiting the number of > > frames they will follow before giving up, so I wasn't sure if I should > > put an arbitrary limit in unwind_frame or just make sure all callers > > are bounded. Your idea of limiting same sp frames instead of total > > frames sounds better. I can send a new patch that adds a new field to > > struct stackframe (which will need to be initialized everywhere, the > > struct is usually on the stack) and limits the recursion. Any > > suggestion on the recursion limit? I would never expect to see a real > > situation with more than a few, but on the other hand parsing the > > frames should be pretty fast so a high number (100?) shouldn't cause > > any user visible effect. > > Talking to some tools guys about this, it sounds like there really > shouldn't be any stackless frame except for the leaf frame. If there are > stackless functions they will probably not be visible in the frame chain > at all. So it might make sense to have a pretty small limit. Maybe it > could even be 1. Cartainly a small number. > > We should also add a check for whether the current and frame and previous > frame appear identical and abort if that's the case, if we don't do that > already. The case that actually worries me is not the "end up looping for ever" case, but the effects of having the stack change while the unwinder is reading from it - for example: /* pop R4-R15 according to mask */ load_sp = mask & (1 << (13 - 4)); while (mask) { if (mask & 1) ctrl->vrs[reg] = *vsp++; mask >>= 1; reg++; } Remember that for a running thread, the stack will be changing all the time while another CPU tries to read it to do the unwind, and also remember that the bottom of stack isn't really known. All you have is the snapshot of the registers when the thread was last stopped by the scheduler, and that state probably isn't valid. So what you're asking is for the unwinder to produce a backtrace from a kernel stack which is possibly changing beneath it from an unknown current state. This doesn't sound like a particularly bright thing to be doing...
On Tue, Oct 16, 2012 at 11:55:04AM +0100, Russell King - ARM Linux wrote: > On Tue, Oct 16, 2012 at 11:12:01AM +0100, Dave Martin wrote: > > On Mon, Oct 15, 2012 at 07:15:31PM -0700, Colin Cross wrote: > > > About half the callers to unwind_frame end up limiting the number of > > > frames they will follow before giving up, so I wasn't sure if I should > > > put an arbitrary limit in unwind_frame or just make sure all callers > > > are bounded. Your idea of limiting same sp frames instead of total > > > frames sounds better. I can send a new patch that adds a new field to > > > struct stackframe (which will need to be initialized everywhere, the > > > struct is usually on the stack) and limits the recursion. Any > > > suggestion on the recursion limit? I would never expect to see a real > > > situation with more than a few, but on the other hand parsing the > > > frames should be pretty fast so a high number (100?) shouldn't cause > > > any user visible effect. > > > > Talking to some tools guys about this, it sounds like there really > > shouldn't be any stackless frame except for the leaf frame. If there are > > stackless functions they will probably not be visible in the frame chain > > at all. So it might make sense to have a pretty small limit. Maybe it > > could even be 1. Cartainly a small number. > > > > We should also add a check for whether the current and frame and previous > > frame appear identical and abort if that's the case, if we don't do that > > already. > > The case that actually worries me is not the "end up looping for ever" > case, but the effects of having the stack change while the unwinder is > reading from it - for example: > > /* pop R4-R15 according to mask */ > load_sp = mask & (1 << (13 - 4)); > while (mask) { > if (mask & 1) > ctrl->vrs[reg] = *vsp++; > mask >>= 1; > reg++; > } > > Remember that for a running thread, the stack will be changing all the > time while another CPU tries to read it to do the unwind, and also > remember that the bottom of stack isn't really known. All you have is > the snapshot of the registers when the thread was last stopped by the > scheduler, and that state probably isn't valid. So long as the unwinder enforces continuous progress towards a fixed limit, sooner or later the supposed bottom of the stack will be reached, or the unwinder will encounter something which is recognised as garbage and stop. This the best we can hope for if trying to print a backtrace for a thread without stopping it... which admittedly seems quite a dodgy thing to attempt. Colin, what are the scenarios when we want to backtrace a thread while it is actually running? > So what you're asking is for the unwinder to produce a backtrace from > a kernel stack which is possibly changing beneath it from an unknown > current state. > > This doesn't sound like a particularly bright thing to be doing... Nonetheless, the changes are relevant to normal stack dumping too, since when we take a fault the sp or stack may be corrupted, even if the thread in question is stopped. Being more robust against infinte loops etc., still seems like a good idea ? Cheers ---Dave
On Tue, Oct 16, 2012 at 3:55 AM, Russell King - ARM Linux <linux@arm.linux.org.uk> wrote: > On Tue, Oct 16, 2012 at 11:12:01AM +0100, Dave Martin wrote: >> On Mon, Oct 15, 2012 at 07:15:31PM -0700, Colin Cross wrote: >> > About half the callers to unwind_frame end up limiting the number of >> > frames they will follow before giving up, so I wasn't sure if I should >> > put an arbitrary limit in unwind_frame or just make sure all callers >> > are bounded. Your idea of limiting same sp frames instead of total >> > frames sounds better. I can send a new patch that adds a new field to >> > struct stackframe (which will need to be initialized everywhere, the >> > struct is usually on the stack) and limits the recursion. Any >> > suggestion on the recursion limit? I would never expect to see a real >> > situation with more than a few, but on the other hand parsing the >> > frames should be pretty fast so a high number (100?) shouldn't cause >> > any user visible effect. >> >> Talking to some tools guys about this, it sounds like there really >> shouldn't be any stackless frame except for the leaf frame. If there are >> stackless functions they will probably not be visible in the frame chain >> at all. So it might make sense to have a pretty small limit. Maybe it >> could even be 1. Cartainly a small number. >> >> We should also add a check for whether the current and frame and previous >> frame appear identical and abort if that's the case, if we don't do that >> already. > > The case that actually worries me is not the "end up looping for ever" > case, but the effects of having the stack change while the unwinder is > reading from it - for example: > > /* pop R4-R15 according to mask */ > load_sp = mask & (1 << (13 - 4)); > while (mask) { > if (mask & 1) > ctrl->vrs[reg] = *vsp++; > mask >>= 1; > reg++; > } > > Remember that for a running thread, the stack will be changing all the > time while another CPU tries to read it to do the unwind, and also > remember that the bottom of stack isn't really known. All you have is > the snapshot of the registers when the thread was last stopped by the > scheduler, and that state probably isn't valid. If the snapshot of the registers when the thread was last stopped includes an sp that points somewhere in two contiguous pages of low memory, I don't see a problem. From the sp we can get the bounds of the stack (see the valid_stack_addr function I added), and we can make sure the unwinder never dereferences anything outside of that stack, so it will never fault. We can also make sure that the sp stays within that stack between frames, and moves in the right direction, so it will never loop (except for the leaf-node sp issue, which Dave Martin's idea will address). > So what you're asking is for the unwinder to produce a backtrace from > a kernel stack which is possibly changing beneath it from an unknown > current state. I don't think the stack changing is relevant. With my modifications, the unwinder can handle an invalid value at any place in the stack without looping or crashing, and it doesn't matter if it is invalid due to changing or permanent stack corruption. The worst it will do is produce a partial stack trace that ends with an invalid value. For example: shell@:/ # dd if=/dev/urandom of=/dev/null bs=1000000 count=1000000 & [1] 2709 130|shell@:/ # while true; do cat /proc/2709/stack; echo ---; done [<c00084d4>] gic_handle_irq+0x24/0x58 [<c000e580>] __irq_svc+0x40/0x70 [<ffffffff>] 0xffffffff --- [<00000099>] 0x99 [<ffffffff>] 0xffffffff --- [<c0039728>] irq_exit+0x7c/0x98 [<c000f888>] handle_IRQ+0x50/0xac [<c00084d4>] gic_handle_irq+0x24/0x58 [<00000014>] 0x14 [<ffffffff>] 0xffffffff --- [<c087ac40>] rcu_preempt_state+0x0/0x140 [<ffffffff>] 0xffffffff --- [<c00084d4>] gic_handle_irq+0x24/0x58 [<c000e580>] __irq_svc+0x40/0x70 [<ffffffff>] 0xffffffff --- [<60000013>] 0x60000013 [<ffffffff>] 0xffffffff --- [<d79ce000>] 0xd79ce000 [<ffffffff>] 0xffffffff > This doesn't sound like a particularly bright thing to be doing... As discussed previously, this already happens, has anyone ever reported it as a problem? Sysrq-t dumps all stacks by calling dump_backtrace(), which bypasses the check for tsk == current. And any caller to unwind_backtrace with preemption on can see a changing stack, even on UP.
On Tue, Oct 16, 2012 at 5:26 AM, Dave Martin <dave.martin@linaro.org> wrote: > On Tue, Oct 16, 2012 at 11:55:04AM +0100, Russell King - ARM Linux wrote: >> On Tue, Oct 16, 2012 at 11:12:01AM +0100, Dave Martin wrote: >> > On Mon, Oct 15, 2012 at 07:15:31PM -0700, Colin Cross wrote: >> > > About half the callers to unwind_frame end up limiting the number of >> > > frames they will follow before giving up, so I wasn't sure if I should >> > > put an arbitrary limit in unwind_frame or just make sure all callers >> > > are bounded. Your idea of limiting same sp frames instead of total >> > > frames sounds better. I can send a new patch that adds a new field to >> > > struct stackframe (which will need to be initialized everywhere, the >> > > struct is usually on the stack) and limits the recursion. Any >> > > suggestion on the recursion limit? I would never expect to see a real >> > > situation with more than a few, but on the other hand parsing the >> > > frames should be pretty fast so a high number (100?) shouldn't cause >> > > any user visible effect. >> > >> > Talking to some tools guys about this, it sounds like there really >> > shouldn't be any stackless frame except for the leaf frame. If there are >> > stackless functions they will probably not be visible in the frame chain >> > at all. So it might make sense to have a pretty small limit. Maybe it >> > could even be 1. Cartainly a small number. >> > >> > We should also add a check for whether the current and frame and previous >> > frame appear identical and abort if that's the case, if we don't do that >> > already. >> >> The case that actually worries me is not the "end up looping for ever" >> case, but the effects of having the stack change while the unwinder is >> reading from it - for example: >> >> /* pop R4-R15 according to mask */ >> load_sp = mask & (1 << (13 - 4)); >> while (mask) { >> if (mask & 1) >> ctrl->vrs[reg] = *vsp++; >> mask >>= 1; >> reg++; >> } >> >> Remember that for a running thread, the stack will be changing all the >> time while another CPU tries to read it to do the unwind, and also >> remember that the bottom of stack isn't really known. All you have is >> the snapshot of the registers when the thread was last stopped by the >> scheduler, and that state probably isn't valid. > > So long as the unwinder enforces continuous progress towards a fixed > limit, sooner or later the supposed bottom of the stack will be reached, > or the unwinder will encounter something which is recognised as garbage > and stop. > > This the best we can hope for if trying to print a backtrace for a > thread without stopping it... which admittedly seems quite a dodgy > thing to attempt. > > Colin, what are the scenarios when we want to backtrace a thread while > it is actually running? There is no case where I want a backtrace from a running thread other than current, but there is no way to guarantee that the thread won't start running unless we stick it in the freezer, which has other problems. The main use case is dumping /proc/pid/stack for all threads in the process when the thread is deadlocked. Most likely none of them will be running, and if they are running I don't care about the stack. If the unwinder is made safe against running tasks, save_stack_trace_tsk could print an error if the task is running (because it has no idea when it will stop running), then capture the stack and test that the thread is still not running and the number of context switches on the task hasn't increased, otherwise retry. That way we'll get an error from a running task, but still be able to dump blocked tasks that are not current. It still relies on being able to call unwind_frame on a possibly invalid stack. >> So what you're asking is for the unwinder to produce a backtrace from >> a kernel stack which is possibly changing beneath it from an unknown >> current state. >> >> This doesn't sound like a particularly bright thing to be doing... > > Nonetheless, the changes are relevant to normal stack dumping too, since > when we take a fault the sp or stack may be corrupted, even if the > thread in question is stopped. Being more robust against infinte loops > etc., still seems like a good idea ? I can split out the unwind_frame hardening from the save_stack_trace_tsk part if you only want that part, and I'll put the save_stack_trace_tsk chunk in the Android tree.
On Tue, Oct 16, 2012 at 02:30:20PM -0700, Colin Cross wrote: > On Tue, Oct 16, 2012 at 3:55 AM, Russell King - ARM Linux > <linux@arm.linux.org.uk> wrote: > > On Tue, Oct 16, 2012 at 11:12:01AM +0100, Dave Martin wrote: > >> On Mon, Oct 15, 2012 at 07:15:31PM -0700, Colin Cross wrote: > >> > About half the callers to unwind_frame end up limiting the number of > >> > frames they will follow before giving up, so I wasn't sure if I should > >> > put an arbitrary limit in unwind_frame or just make sure all callers > >> > are bounded. Your idea of limiting same sp frames instead of total > >> > frames sounds better. I can send a new patch that adds a new field to > >> > struct stackframe (which will need to be initialized everywhere, the > >> > struct is usually on the stack) and limits the recursion. Any > >> > suggestion on the recursion limit? I would never expect to see a real > >> > situation with more than a few, but on the other hand parsing the > >> > frames should be pretty fast so a high number (100?) shouldn't cause > >> > any user visible effect. > >> > >> Talking to some tools guys about this, it sounds like there really > >> shouldn't be any stackless frame except for the leaf frame. If there are > >> stackless functions they will probably not be visible in the frame chain > >> at all. So it might make sense to have a pretty small limit. Maybe it > >> could even be 1. Cartainly a small number. > >> > >> We should also add a check for whether the current and frame and previous > >> frame appear identical and abort if that's the case, if we don't do that > >> already. > > > > The case that actually worries me is not the "end up looping for ever" > > case, but the effects of having the stack change while the unwinder is > > reading from it - for example: > > > > /* pop R4-R15 according to mask */ > > load_sp = mask & (1 << (13 - 4)); > > while (mask) { > > if (mask & 1) > > ctrl->vrs[reg] = *vsp++; > > mask >>= 1; > > reg++; > > } > > > > Remember that for a running thread, the stack will be changing all the > > time while another CPU tries to read it to do the unwind, and also > > remember that the bottom of stack isn't really known. All you have is > > the snapshot of the registers when the thread was last stopped by the > > scheduler, and that state probably isn't valid. > > If the snapshot of the registers when the thread was last stopped > includes an sp that points somewhere in two contiguous pages of low > memory, I don't see a problem. From the sp we can get the bounds of > the stack (see the valid_stack_addr function I added), and we can make > sure the unwinder never dereferences anything outside of that stack, > so it will never fault. We can also make sure that the sp stays > within that stack between frames, and moves in the right direction, so > it will never loop (except for the leaf-node sp issue, which Dave > Martin's idea will address). > > > So what you're asking is for the unwinder to produce a backtrace from > > a kernel stack which is possibly changing beneath it from an unknown > > current state. > > I don't think the stack changing is relevant. With my modifications, > the unwinder can handle an invalid value at any place in the stack > without looping or crashing, and it doesn't matter if it is invalid > due to changing or permanent stack corruption. The worst it will do > is produce a partial stack trace that ends with an invalid value. For > example: > > shell@:/ # dd if=/dev/urandom of=/dev/null bs=1000000 count=1000000 & > [1] 2709 > 130|shell@:/ # while true; do cat /proc/2709/stack; echo ---; done > [<c00084d4>] gic_handle_irq+0x24/0x58 > [<c000e580>] __irq_svc+0x40/0x70 > [<ffffffff>] 0xffffffff > --- > [<00000099>] 0x99 > [<ffffffff>] 0xffffffff > --- > [<c0039728>] irq_exit+0x7c/0x98 > [<c000f888>] handle_IRQ+0x50/0xac > [<c00084d4>] gic_handle_irq+0x24/0x58 > [<00000014>] 0x14 > [<ffffffff>] 0xffffffff > --- > [<c087ac40>] rcu_preempt_state+0x0/0x140 > [<ffffffff>] 0xffffffff > --- > [<c00084d4>] gic_handle_irq+0x24/0x58 > [<c000e580>] __irq_svc+0x40/0x70 > [<ffffffff>] 0xffffffff > --- > [<60000013>] 0x60000013 > [<ffffffff>] 0xffffffff > --- > [<d79ce000>] 0xd79ce000 > [<ffffffff>] 0xffffffff > > > This doesn't sound like a particularly bright thing to be doing... > > As discussed previously, this already happens, has anyone ever > reported it as a problem? Sysrq-t dumps all stacks by calling > dump_backtrace(), which bypasses the check for tsk == current. And > any caller to unwind_backtrace with preemption on can see a changing > stack, even on UP. I think I agree with that view: so long as we are just adding robustness against garbage stacks I think the proposed changes are useful anyway. A changing stack is just one kind of garbage. We don't have to guarantee a sensible backtrace in that case, so long as the unwinder executes safely and doesn't loop. Cheers ---Dave
diff --git a/arch/arm/kernel/stacktrace.c b/arch/arm/kernel/stacktrace.c index 45e6b7e..f51dd68 100644 --- a/arch/arm/kernel/stacktrace.c +++ b/arch/arm/kernel/stacktrace.c @@ -105,23 +105,11 @@ void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace) data.skip = trace->skip; if (tsk != current) { -#if defined(CONFIG_SMP) || \ - (defined(CONFIG_FRAME_POINTER) && !defined(CONFIG_ARM_UNWIND)) - /* - * What guarantees do we have here that 'tsk' is not - * running on another CPU? For now, ignore it as we - * can't guarantee we won't explode. - */ - if (trace->nr_entries < trace->max_entries) - trace->entries[trace->nr_entries++] = ULONG_MAX; - return; -#else data.no_sched_functions = 1; frame.fp = thread_saved_fp(tsk); frame.sp = thread_saved_sp(tsk); frame.lr = 0; /* recovered from the stack */ frame.pc = thread_saved_pc(tsk); -#endif } else { register unsigned long current_sp asm ("sp"); diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c 00df012..b3a09ad 100644 --- a/arch/arm/kernel/unwind.c +++ b/arch/arm/kernel/unwind.c @@ -98,6 +98,16 @@ enum regs { (unsigned long)(ptr) + offset; \ }) +static bool valid_stack_addr(unsigned long sp, unsigned long *vsp) +{ + unsigned long low; + unsigned long high; + + low = round_down(sp, THREAD_SIZE) + sizeof(struct thread_info); + high = ALIGN(sp, THREAD_SIZE); + return ((unsigned long)vsp >= low && (unsigned long)vsp < high); +} + /* * Binary search in the unwind index. The entries are * guaranteed to be sorted in ascending order by the linker. @@ -241,6 +251,7 @@ static unsigned long unwind_get_byte(struct unwind_ctrl_block *ctrl) static int unwind_exec_insn(struct unwind_ctrl_block *ctrl) { unsigned long insn = unwind_get_byte(ctrl); + unsigned long orig_sp = ctrl->vrs[SP]; pr_debug("%s: insn = %08lx\n", __func__, insn); @@ -264,8 +275,11 @@ static int unwind_exec_insn(struct unwind_ctrl_block *ctrl) /* pop R4-R15 according to mask */ load_sp = mask & (1 << (13 - 4)); while (mask) { - if (mask & 1) + if (mask & 1) { + if (!valid_stack_addr(orig_sp, vsp)) + return -URC_FAILURE; ctrl->vrs[reg] = *vsp++; + } mask >>= 1; reg++; } @@ -279,10 +293,16 @@ static int unwind_exec_insn(struct unwind_ctrl_block *ctrl) int reg; /* pop R4-R[4+bbb] */ - for (reg = 4; reg <= 4 + (insn & 7); reg++) + for (reg = 4; reg <= 4 + (insn & 7); reg++) { + if (!valid_stack_addr(orig_sp, vsp)) + return -URC_FAILURE; ctrl->vrs[reg] = *vsp++; - if (insn & 0x80) + } + if (insn & 0x80) { + if (!valid_stack_addr(orig_sp, vsp)) + return -URC_FAILURE; ctrl->vrs[14] = *vsp++; + } ctrl->vrs[SP] = (unsigned long)vsp; } else if (insn == 0xb0) { if (ctrl->vrs[PC] == 0) @@ -302,8 +322,11 @@ static int unwind_exec_insn(struct unwind_ctrl_block *ctrl) /* pop R0-R3 according to mask */ while (mask) { - if (mask & 1) + if (mask & 1) { + if (!valid_stack_addr(orig_sp, vsp)) + return -URC_FAILURE; ctrl->vrs[reg] = *vsp++; + } mask >>= 1; reg++; }
Unwinding with CONFIG_ARM_UNWIND is much more complicated than unwinding with CONFIG_FRAME_POINTER, but there are only a few points that require validation in order to avoid faults or infinite loops. Avoiding faults is easy by adding checks to verify that all accesses relative to the frame's stack pointer remain inside the stack. When CONFIG_FRAME_POINTER is not set it is possible for two frames to have the same SP, so there is no way to avoid repeated calls to unwind_frame continuing forever. Signed-off-by: Colin Cross <ccross@android.com> --- arch/arm/kernel/stacktrace.c | 12 ------------ arch/arm/kernel/unwind.c | 31 +++++++++++++++++++++++++++---- 2 files changed, 27 insertions(+), 16 deletions(-)