[PATCH] x86-64: Align the stack in __tls_get_addr [BZ #21609]

[Florian Weimer]

On 06/19/2017 12:18 PM, Florian Weimer wrote:
> This is a follow-up to bug 15128, where an unaligned stack was
> compensated for in the dynamic linker relocation trampoline.
> __tls_get_addr itself was not fixed, though, and the function can reach
> deeply into libc due to its malloc dependency, so can result crashes
> with an unaligned stack, too.
> 
> The attached patch adds a compatibility implementation of __tls_get_addr
> which aligns the stack, but only on the slow path.  Internal calls go to
> the default implementation (from elf/dl-tls.c) which does not perform
> stack realignment.
> 
> I plan to submit a follow-up patch which adds a new symbol version for
> __tls_get_addr which bypasses the stack alignment for new binaries.
> 
> In my patch, the CFI annotations need review.  I have never written
> those before.

I went over the CFI annotations with Jakub.  He suggested some
simplifications, incorporated in the attached patch.  We don't need
.cfi_restore because the %rbp value at the specified offset is protected
by the red zone until the function returns.

Thanks,
Florian
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