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linux-next/drivers/misc/lkdtm_rodata.c
Michael Ellerman c012268b37 lkdtm: Mark lkdtm_rodata_do_nothing() notrace
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>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2016-08-31 13:12:38 +02:00

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C

/*
* This includes functions that are meant to live entirely in .rodata
* (via objcopy tricks), to validate the non-executability of .rodata.
*/
#include "lkdtm.h"
void notrace lkdtm_rodata_do_nothing(void)
{
/* Does nothing. We just want an architecture agnostic "return". */
}