| Message ID | 1470200371-31565-1-git-send-email-mpe@ellerman.id.au (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Tue, Aug 2, 2016 at 9:59 PM, Michael Ellerman <mpe@ellerman.id.au> wrote: > lkdtm_rodata_do_nothing() is an empty function which is generated in > order to test the non-executability of rodata. > > Currently if function tracing is enabled then an mcount callsite will be > generated for lkdtm_rodata_do_nothing(), and it will appear in the list > of available functions for function tracing (available_filter_functions). > > Given it's purpose purely as a test function, it seems preferable for > lkdtm_rodata_do_nothing() to be marked notrace, so it doesn't appear as > traceable. > > This also avoids triggering a linker bug on powerpc: > > https://sourceware.org/bugzilla/show_bug.cgi?id=20428 > > When the linker sees code that needs to generate a call stub, eg. a > branch to mcount(), it assumes the section is executable and > dereferences a NULL pointer leading to a linker segfault. Marking > lkdtm_rodata_do_nothing() notrace avoids triggering the bug because the > function contains no other function calls. > > Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> Awesome! Thanks for tracking this down. I've applied it to my tree, it should get picked up by Greg on my next pull request. -Kees > --- > drivers/misc/lkdtm_rodata.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/misc/lkdtm_rodata.c b/drivers/misc/lkdtm_rodata.c > index 166b1db3969f..3564477b8c2d 100644 > --- a/drivers/misc/lkdtm_rodata.c > +++ b/drivers/misc/lkdtm_rodata.c > @@ -4,7 +4,7 @@ > */ > #include "lkdtm.h" > > -void lkdtm_rodata_do_nothing(void) > +void notrace lkdtm_rodata_do_nothing(void) > { > /* Does nothing. We just want an architecture agnostic "return". */ > } > -- > 2.7.4 >
diff --git a/drivers/misc/lkdtm_rodata.c b/drivers/misc/lkdtm_rodata.c index 166b1db3969f..3564477b8c2d 100644 --- a/drivers/misc/lkdtm_rodata.c +++ b/drivers/misc/lkdtm_rodata.c @@ -4,7 +4,7 @@ */ #include "lkdtm.h" -void lkdtm_rodata_do_nothing(void) +void notrace lkdtm_rodata_do_nothing(void) { /* Does nothing. We just want an architecture agnostic "return". */ }
lkdtm_rodata_do_nothing() is an empty function which is generated in order to test the non-executability of rodata. Currently if function tracing is enabled then an mcount callsite will be generated for lkdtm_rodata_do_nothing(), and it will appear in the list of available functions for function tracing (available_filter_functions). Given it's purpose purely as a test function, it seems preferable for lkdtm_rodata_do_nothing() to be marked notrace, so it doesn't appear as traceable. This also avoids triggering a linker bug on powerpc: https://sourceware.org/bugzilla/show_bug.cgi?id=20428 When the linker sees code that needs to generate a call stub, eg. a branch to mcount(), it assumes the section is executable and dereferences a NULL pointer leading to a linker segfault. Marking lkdtm_rodata_do_nothing() notrace avoids triggering the bug because the function contains no other function calls. Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> --- drivers/misc/lkdtm_rodata.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)