| Message ID | 20160414093337.GD16549@mwanda (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Thu, Apr 14, 2016 at 12:33:37PM +0300, Dan Carpenter wrote: > It acpi_acquire_global_lock() return AE_NOT_CONFIGURED then "glk" isn't ^ ^ If returns > initialized, which, if you got very unlucky, could cause a bug. In principle I'm OK with being cautious and initializing local variables just in case, but I'm curious: acpi_acquire_global_lock() (and its friend, acpi_release_global_lock()) are both wrapped inside the same macro -- ACPI_HW_DEPENDENT_RETURN_STATUS -- which either makes them both do something useful, or makes them both no-ops returning a hardcoded AE_NOT_CONFIGURED. So what else do you think could be a way to get very unlucky ? Otherwise, sure: Just 'cause we're paranoid doesn't mean someone's not out to get us! :) Thanks, --Gabriel > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com> > > diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c > index d999fe3..0e20116 100644 > --- a/drivers/firmware/qemu_fw_cfg.c > +++ b/drivers/firmware/qemu_fw_cfg.c > @@ -77,7 +77,7 @@ static inline u16 fw_cfg_sel_endianness(u16 key) > static inline void fw_cfg_read_blob(u16 key, > void *buf, loff_t pos, size_t count) > { > - u32 glk; > + u32 glk = -1U; > acpi_status status; > > /* If we have ACPI, ensure mutual exclusion against any potential
On Thu, Apr 14, 2016 at 02:40:06PM -0400, Gabriel L. Somlo wrote: > On Thu, Apr 14, 2016 at 12:33:37PM +0300, Dan Carpenter wrote: > > It acpi_acquire_global_lock() return AE_NOT_CONFIGURED then "glk" isn't > ^ ^ > If returns > > > initialized, which, if you got very unlucky, could cause a bug. > > > In principle I'm OK with being cautious and initializing local > variables just in case, but I'm curious: > > acpi_acquire_global_lock() (and its friend, acpi_release_global_lock()) > are both wrapped inside the same macro -- ACPI_HW_DEPENDENT_RETURN_STATUS > -- which either makes them both do something useful, or makes them both > no-ops returning a hardcoded AE_NOT_CONFIGURED. > > So what else do you think could be a way to get very unlucky ? If "glk" happened to to equal acpi_gbl_global_lock_handle by chance then we would release it without acquiring it first. Actually I could initialize it to zero and that would be better, no? regards, dan carpenter
On Thu, Apr 14, 2016 at 10:12:53PM +0300, Dan Carpenter wrote: > On Thu, Apr 14, 2016 at 02:40:06PM -0400, Gabriel L. Somlo wrote: > > On Thu, Apr 14, 2016 at 12:33:37PM +0300, Dan Carpenter wrote: > > > It acpi_acquire_global_lock() return AE_NOT_CONFIGURED then "glk" isn't > > ^ ^ > > If returns > > > > > initialized, which, if you got very unlucky, could cause a bug. > > > > > > In principle I'm OK with being cautious and initializing local > > variables just in case, but I'm curious: > > > > acpi_acquire_global_lock() (and its friend, acpi_release_global_lock()) > > are both wrapped inside the same macro -- ACPI_HW_DEPENDENT_RETURN_STATUS > > -- which either makes them both do something useful, or makes them both > > no-ops returning a hardcoded AE_NOT_CONFIGURED. > > > > So what else do you think could be a way to get very unlucky ? > > If "glk" happened to to equal acpi_gbl_global_lock_handle by chance > then we would release it without acquiring it first. Actually I could > initialize it to zero and that would be better, no? No, because acpi_release_global_lock() would also be a hard-coded "return AE_NOT_CONFIGURED" by the same macro which also hard-coded acpi_acquire_global_lock() to be "return AE_NOT_CONFIGURED" in the first place. See include/acpi/acpixf.h, search for the two occurrences of "#define ACPI_HW_DEPENDENT_RETURN_STATUS" and then for: "global_lock" further down in the file. Whether both (or neither) of lock/unlock are for real or just hardcoded to return AE_NOT_CONFIGURED depends on ACPI_REDUCED_HARDWARE, which I assume is also set when there's *no* ACPI hardware at all. But I don't believe it's possible for "unlock" to do anything at all if "lock" was hardcoded to simply return AE_NOT_CONFIGURED. Then again, it's possible I'm still missing something :) Thanks, --Gabe
On Thu, Apr 14, 2016 at 12:33:37PM +0300, Dan Carpenter wrote: > It acpi_acquire_global_lock() return AE_NOT_CONFIGURED then "glk" isn't > initialized, which, if you got very unlucky, could cause a bug. > > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com> > > diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c > index d999fe3..0e20116 100644 > --- a/drivers/firmware/qemu_fw_cfg.c > +++ b/drivers/firmware/qemu_fw_cfg.c > @@ -77,7 +77,7 @@ static inline u16 fw_cfg_sel_endianness(u16 key) > static inline void fw_cfg_read_blob(u16 key, > void *buf, loff_t pos, size_t count) > { > - u32 glk; > + u32 glk = -1U; After digging through the acpi_[acquire|release]_global_lock() code in drivers/acpi/acpica/evxface.c, the -1 value actually makes sense, as glk is set to the value of acpi_gbl_global_lock_handle, which internally is a 16-bit value which can wrap around, but will never be equal to 32-bit "-1". As such, the unlock function would fail with AE_NOT_ACQUIRED if its "for-real" version ever ended up being called. So, with the typos in the commit blurb fixed (s/It/If/ and s/return/returns/), and on general "belt-and-suspenders" principle, Reviewed-by: Gabriel Somlo <somlo@cmu.edu> I just wanted to make sure my understanding of "this can't happen with the way the ACPI macros are currently defined" is still correct :) Thanks, --Gabe > acpi_status status; > > /* If we have ACPI, ensure mutual exclusion against any potential
Ah... I see now. You're right. Thanks for the explanation. On my config those functions are no-ops so the variable isn't initialized. If they were enabled then *probably* it wouldn't generate a warning. Probably just silencing the warning is the way to go though... I bet GCC optimizes it away. Let me think about this some more... regards, dan carpenter
diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c index d999fe3..0e20116 100644 --- a/drivers/firmware/qemu_fw_cfg.c +++ b/drivers/firmware/qemu_fw_cfg.c @@ -77,7 +77,7 @@ static inline u16 fw_cfg_sel_endianness(u16 key) static inline void fw_cfg_read_blob(u16 key, void *buf, loff_t pos, size_t count) { - u32 glk; + u32 glk = -1U; acpi_status status; /* If we have ACPI, ensure mutual exclusion against any potential
It acpi_acquire_global_lock() return AE_NOT_CONFIGURED then "glk" isn't initialized, which, if you got very unlucky, could cause a bug. Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>