| Message ID | 20181113130730.44844-1-namit@vmware.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | x86/alternative: text_poke() fixes | expand |
On Tue, Nov 13, 2018 at 05:07:20AM -0800, Nadav Amit wrote: > v4->v5: > - Fix Xen breakage [Damian Tometzki] > - BUG_ON() when poking_mm initialization fails [PeterZ] > - Better comments on "x86/mm: temporary mm struct" > - Cleaner removal of the custom poker I'll re-iterate my position: it is impossible for the text not to match, and if it somehow does not match, something went sideways in an unrecoverably fashion. text_poke() must not fail, ever. If it does, our text is inconsistent and we must abort/panic/bug. The only way I will accept anything else is if someone can come up with a sensible scenario of text_poke() failing and recovering from it. AFAICT there is no possible way to gracefully recover. Consider a jump label with multiple patch sites; we patch the first, then fail. In order to restore to a sane state, we must undo the patching of the first, but undoing text_poke() fails again. Then what? Allowing text_poke() to fail only creates an unfixable mess. Esp. since there is no sane scenario under which is can fail. --- --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -695,7 +695,7 @@ void __init_or_module text_poke_early(vo __ro_after_init struct mm_struct *poking_mm; __ro_after_init unsigned long poking_addr; -static int __text_poke(void *addr, const void *opcode, size_t len) +static void __text_poke(void *addr, const void *opcode, size_t len) { bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE; temporary_mm_state_t prev; @@ -731,13 +731,10 @@ static int __text_poke(void *addr, const * The lock is not really needed, but this allows to avoid open-coding. */ ptep = get_locked_pte(poking_mm, poking_addr, &ptl); - /* - * If we failed to allocate a PTE, fail. This should *never* happen, - * since we preallocate the PTE. + * This must not fail; preallocated in poking_init(). */ - if (WARN_ON_ONCE(!ptep)) - goto out; + VM_BUG_ON(!ptep) pte = mk_pte(pages[0], PAGE_KERNEL); set_pte_at(poking_mm, poking_addr, ptep, pte); @@ -795,12 +792,14 @@ static int __text_poke(void *addr, const unuse_temporary_mm(prev); pte_unmap_unlock(ptep, ptl); -out: - if (memcmp(addr, opcode, len)) - r = -EFAULT; + + /* + * If the text doesn't match what we just wrote; something is + * fundamentally screwy, there's nothing we can really do about that. + */ + BUG_ON(memcmp(addr, opcode, len)); local_irq_restore(flags); - return r; } /** @@ -814,21 +813,10 @@ static int __text_poke(void *addr, const * in a way that permits an atomic write. It also makes sure we fit on a single * page. */ -int text_poke(void *addr, const void *opcode, size_t len) +void text_poke(void *addr, const void *opcode, size_t len) { - int r; - lockdep_assert_held(&text_mutex); - - r = __text_poke(addr, opcode, len); - - /* - * TODO: change the callers to consider the return value and remove this - * historical assertion. - */ - BUG_ON(r); - - return r; + __text_poke(addr, opcode, len); } /** @@ -847,7 +835,7 @@ int text_poke(void *addr, const void *op */ int text_poke_kgdb(void *addr, const void *opcode, size_t len) { - return __text_poke(addr, opcode, len); + __text_poke(addr, opcode, len); } static void do_sync_core(void *info) --- a/arch/x86/kernel/kgdb.c +++ b/arch/x86/kernel/kgdb.c @@ -767,10 +767,8 @@ int kgdb_arch_set_breakpoint(struct kgdb */ if (mutex_is_locked(&text_mutex)) return -EBUSY; - err = text_poke_kgdb((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr, + text_poke_kgdb((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr, BREAK_INSTR_SIZE); - if (err) - return err; bpt->type = BP_POKE_BREAKPOINT; return err; @@ -788,11 +786,8 @@ int kgdb_arch_remove_breakpoint(struct k */ if (mutex_is_locked(&text_mutex)) goto knl_write; - err = text_poke_kgdb((void *)bpt->bpt_addr, bpt->saved_instr, - BREAK_INSTR_SIZE); - if (err) - goto knl_write; - return err; + text_poke_kgdb((void *)bpt->bpt_addr, bpt->saved_instr, BREAK_INSTR_SIZE); + return 0; knl_write: return probe_kernel_write((char *)bpt->bpt_addr,
> On Nov 20, 2018, at 4:42 AM, Peter Zijlstra <peterz@infradead.org> wrote: > > On Tue, Nov 13, 2018 at 05:07:20AM -0800, Nadav Amit wrote: >> v4->v5: >> - Fix Xen breakage [Damian Tometzki] >> - BUG_ON() when poking_mm initialization fails [PeterZ] >> - Better comments on "x86/mm: temporary mm struct" >> - Cleaner removal of the custom poker > > I'll re-iterate my position: it is impossible for the text not to match, > and if it somehow does not match, something went sideways in an > unrecoverably fashion. > > text_poke() must not fail, ever. If it does, our text is inconsistent > and we must abort/panic/bug. > > The only way I will accept anything else is if someone can come up with > a sensible scenario of text_poke() failing and recovering from it. > AFAICT there is no possible way to gracefully recover. > > Consider a jump label with multiple patch sites; we patch the first, > then fail. In order to restore to a sane state, we must undo the > patching of the first, but undoing text_poke() fails again. Then > what? > > Allowing text_poke() to fail only creates an unfixable mess. Esp. since > there is no sane scenario under which is can fail. Ok, ok... I tried to stand my ground, but I guess I failed. I don’t feel that strongly about this assertion to argue with you. I’m just the “chicken” kind of guy. Yet, take into consideration that I will need to use you as my “vest” once I get being “shot” for adding BUG_ON(). ;-) I will send another version tonight, assuming no new issues are raised. Regards, NAdav