Re: [PATCH] sparc: Remove unwind information from signal return stub functions

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 12/01/24 06:26, Daniel Cederman wrote:
> The functions were previously written in C, but were not compiled
> with unwind information. The ENTRY/END macros includes .cfi_startproc
> and .cfi_endproc which adds unwind information. This caused the
> tests cleanup-8 and cleanup-10 in the GCC testsuite to fail.
> This patch adds a version of the ENTRY/END macros without the
> CFI instructions that can be used instead.
> 
> sigaction registers a restorer address that is located two instructions
> before the stub function. This patch adds a two instruction padding to
> avoid that the unwinder accesses the unwind information from the function
> that the linker has placed right before it in memory. This fixes an issue
> with pthread_cancel that caused tst-mutex8-static (and other tests) to fail.
> 
> This patch fixes the issues I have seen, but I am not sure if this is
> the right solution, so any feedback is welcome!
> 
> Signed-off-by: Daniel Cederman <cederman@gaisler.com>

I am not sure this is the correct fix, at least on a sparc machine I 
do not any improvements on a sparcv9 build. But do I agree that this
seems to be a wrong/missing unwind information for the signal return
stub: the tst-mutex8-static does seems be on an infinite loop in the
libgcc unwinder code.

Does it help to restore the stub to be coded in C? It seems that the
problem that lead to rewrite the stubs in assembly (b33e946fbb1659d)
is mainly caused by '-fPIE/-fpic/-fPIC' which seems to generate a
stack frame.  I see it has started with gcc 8, and I am not sure if
this is really required by sparc ABI or it is something related to
to other compiler flags we are using (I tried to use
-fno-asynchronous-unwind-tables -fno-exceptions to prevent it, but
without much success).


> ---
>  sysdeps/sparc/sysdep.h                                |  9 +++++++++
>  .../unix/sysv/linux/sparc/sparc32/sigreturn_stub.S    | 11 +++++++----
>  2 files changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/sysdeps/sparc/sysdep.h b/sysdeps/sparc/sysdep.h
> index c00fe79fec..44a6952bff 100644
> --- a/sysdeps/sparc/sysdep.h
> +++ b/sysdeps/sparc/sysdep.h
> @@ -76,6 +76,15 @@ C_LABEL(name)				\
>  	cfi_endproc;			\
>  	.size name, . - name
>  
> +#define ENTRY_NOCFI(name)			\
> +	.align	4;			\
> +	.global	C_SYMBOL_NAME(name);	\
> +	.type	name, @function;	\
> +C_LABEL(name)
> +
> +#define END_NOCFI(name)			\
> +	.size name, . - name
> +
>  #undef LOC
>  #define LOC(name)  .L##name
>  
> diff --git a/sysdeps/unix/sysv/linux/sparc/sparc32/sigreturn_stub.S b/sysdeps/unix/sysv/linux/sparc/sparc32/sigreturn_stub.S
> index 832223f8ce..21d36c50df 100644
> --- a/sysdeps/unix/sysv/linux/sparc/sparc32/sigreturn_stub.S
> +++ b/sysdeps/unix/sysv/linux/sparc/sparc32/sigreturn_stub.S
> @@ -23,12 +23,15 @@
>  
>     [1] https://lkml.org/lkml/2016/5/27/465  */
>  
> -ENTRY (__rt_sigreturn_stub)
> +	nop
> +	nop
> +
> +ENTRY_NOCFI (__rt_sigreturn_stub)
>  	mov	__NR_rt_sigreturn, %g1
>  	ta	0x10
> -END (__rt_sigreturn_stub)
> +END_NOCFI (__rt_sigreturn_stub)
>  
> -ENTRY (__sigreturn_stub)
> +ENTRY_NOCFI (__sigreturn_stub)
>  	mov	__NR_sigreturn, %g1
>  	ta	0x10
> -END (__sigreturn_stub)
> +END_NOCFI (__sigreturn_stub)