[PATCH 2/2] Revert "x86_64: Remove unused padding from tcbhead_t"

[Adhemerval Zanella Netto]

On 04/01/25 10:19, Florian Weimer wrote:
> This reverts commit 30d3fd7f4f4bc8f767d73ad4e4b005c1bd234310.
> 
> The padding is required by Chromium's MaybeUpdateGlibcTidCache
> in sandbox/linux/services/namespace_sandbox.cc.

LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>


> ---
>  sysdeps/x86_64/nptl/tls.h | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/sysdeps/x86_64/nptl/tls.h b/sysdeps/x86_64/nptl/tls.h
> index a9c348e696..551d342d52 100644
> --- a/sysdeps/x86_64/nptl/tls.h
> +++ b/sysdeps/x86_64/nptl/tls.h
> @@ -62,16 +62,28 @@ typedef struct
>    void *__private_ss;
>    /* The marker for the current shadow stack.  */
>    unsigned long long int ssp_base;
> +  /* Must be kept even if it is no longer used by glibc since programs,
> +     like AddressSanitizer, depend on the size of tcbhead_t.  */
> +  __128bits __glibc_unused2[8][4] __attribute__ ((aligned (32)));
> +
> +  void *__padding[8];
>  } tcbhead_t;
>  
>  # ifdef __ILP32__
>  /* morestack.S in libgcc uses offset 0x40 to access __private_ss,   */
>  _Static_assert (offsetof (tcbhead_t, __private_ss) == 0x40,
>  		"offset of __private_ss != 0x40");
> +/* NB: ssp_base used to be "long int __glibc_reserved2", which was
> +   changed from 32 bits to 64 bits.  Make sure that the offset of the
> +   next field, __glibc_unused2, is unchanged.  */
> +_Static_assert (offsetof (tcbhead_t, __glibc_unused2) == 0x60,
> +		"offset of __glibc_unused2 != 0x60");
>  # else
>  /* morestack.S in libgcc uses offset 0x70 to access __private_ss,   */
>  _Static_assert (offsetof (tcbhead_t, __private_ss) == 0x70,
>  		"offset of __private_ss != 0x70");
> +_Static_assert (offsetof (tcbhead_t, __glibc_unused2) == 0x80,
> +		"offset of __glibc_unused2 != 0x80");
>  # endif
>  
>  #else /* __ASSEMBLER__ */