Re: [PATCH v2] MIPS: Avoid VDSO ABI breakage due to global register variable

[Paul Burton <paulburton@kernel.org>]

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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