Hello,
Paul Burton wrote:
Declaring __current_thread_info as a global register variable has the effect of preventing GCC from saving & restoring its value in cases where the ABI would typically do so.
To quote GCC documentation:
If the register is a call-saved register, call ABI is affected: the register will not be restored in function epilogue sequences after the variable has been assigned. Therefore, functions cannot safely return to callers that assume standard ABI.
When our position independent VDSO is built for the n32 or n64 ABIs all functions it exposes should be preserving the value of $gp/$28 for their caller, but in the presence of the __current_thread_info global register variable GCC stops doing so & simply clobbers $gp/$28 when calculating the address of the GOT.
In cases where the VDSO returns success this problem will typically be masked by the caller in libc returning & restoring $gp/$28 itself, but that is by no means guaranteed. In cases where the VDSO returns an error libc will typically contain a fallback path which will now fail (typically with a bad memory access) if it attempts anything which relies upon the value of $gp/$28 - eg. accessing anything via the GOT.
One fix for this would be to move the declaration of __current_thread_info inside the current_thread_info() function, demoting it from global register variable to local register variable & avoiding inadvertently creating a non-standard calling ABI for the VDSO. Unfortunately this causes issues for clang, which doesn't support local register variables as pointed out by commit fe92da0f355e ("MIPS: Changed current_thread_info() to an equivalent supported by both clang and GCC") which introduced the global register variable before we had a VDSO to worry about.
Instead, fix this by continuing to use the global register variable for the kernel proper but declare __current_thread_info as a simple extern variable when building the VDSO. It should never be referenced, and will cause a link error if it is. This resolves the calling convention issue for the VDSO without having any impact upon the build of the kernel itself for either clang or gcc.
Applied to mips-fixes.
commit bbcc5672b006 https://git.kernel.org/mips/c/bbcc5672b006
Signed-off-by: Paul Burton paulburton@kernel.org Fixes: ebb5e78cc634 ("MIPS: Initial implementation of a VDSO") Reported-by: Jason A. Donenfeld Jason@zx2c4.com Reviewed-by: Jason A. Donenfeld Jason@zx2c4.com Tested-by: Jason A. Donenfeld Jason@zx2c4.com
Thanks, Paul
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