[PATCH v2] Mark swapcontext returns_twice by default [BZ #23130]

[Adhemerval Zanella Netto]

On 04/09/26 12:29, Stian Halseth wrote:
> swapcontext does not restore every callee-saved register on all targets.
> On sparc the %l and %i registers are not in the saved context at all:
> they live in the register window save area of the calling frame, which
> another pass through that frame can overwrite.  stdlib/tst-setcontext2
> fails on sparc64 for this reason at -O2 with PIC, where gcc keeps
> &global in %i5 across swapcontext and reuses %i5 as the GOT base on
> another path through the same frame.
> 
> Mark swapcontext returns_twice in the generic bits/indirect-return.h, so
> every target that does not override it is covered.  x86 and aarch64 keep
> their own headers and are unchanged.
> 
> string/tst-xbzero-opt.c calls swapcontext from an always_inline
> function, which a returns_twice callee forbids; key its guard on the
> availability of indirect_return rather than on __CET__.  It is the only
> such site in the tree.
> 
> Tested on sparc64: full make check is clean, with no test that passed
> before failing after.
> 
> Signed-off-by: Stian Halseth <stian@itx.no>


LGTM, thanks.

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

> ---
> Changes in v2:
>  - drop the sparc-specific rationale from the generic header
>  - use returns_twice unconditionally rather than preferring
>    indirect_return
> 
> I left the tst-xbzero-opt.c guard keyed on indirect_return.  The
> attribute is x86/aarch64-only in practice -- __has_attribute reports 1
> on x86_64 and 0 on sparc64 -- and both of those targets have their own
> headers, so the guard still selects exactly the targets that end up with
> returns_twice.
> 
> On sparc64 the generic header resolves to returns_twice either way, so
> the make check result from v1 stands unchanged.
> 
>  bits/indirect-return.h  | 9 +++++++--
>  string/tst-xbzero-opt.c | 7 +++----
>  2 files changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/bits/indirect-return.h b/bits/indirect-return.h
> index 819bd25b..ba2efb06 100644
> --- a/bits/indirect-return.h
> +++ b/bits/indirect-return.h
> @@ -21,5 +21,10 @@
>  #endif
>  
>  /* __INDIRECT_RETURN is used on swapcontext to indicate if it requires
> -   special compiler treatment.  */
> -#define __INDIRECT_RETURN
> +   special compiler treatment.  swapcontext does not restore every
> +   callee-saved register on all targets.  */
> +#if __glibc_has_attribute (__returns_twice__)
> +# define __INDIRECT_RETURN __attribute__ ((__returns_twice__))
> +#else
> +# define __INDIRECT_RETURN
> +#endif
> diff --git a/string/tst-xbzero-opt.c b/string/tst-xbzero-opt.c
> index 3aa13d03..26e2ec1c 100644
> --- a/string/tst-xbzero-opt.c
> +++ b/string/tst-xbzero-opt.c
> @@ -111,10 +111,9 @@ use_test_buffer (unsigned char *buf)
>  /* Always check the test buffer immediately after filling it; this
>     makes externally visible side effects depend on the buffer existing
>     and having been filled in.  */
> -#if defined __CET__ && !__glibc_has_attribute (__indirect_return__)
> -/* Note: swapcontext returns via indirect branch when SHSTK is enabled.
> -   Without indirect_return attribute, swapcontext is marked with
> -   returns_twice attribute, which prevents always_inline to work.  */
> +#if !__glibc_has_attribute (__indirect_return__)
> +/* Without indirect_return, swapcontext is marked returns_twice, which
> +   prevents always_inline from working.  */
>  # define ALWAYS_INLINE
>  #else
>  # define ALWAYS_INLINE	__attribute__ ((always_inline))