Re: [PATCH] powerpc: Don't drop _TIF_RESTOREALL on syscall restart

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 8/29/26 9:49 AM, Ritesh Harjani (IBM) wrote:
> So the syscall return sequence is as follows:
> A syscall return to userspace is prepared and then a short asm sequence
> that actually does the RFI. Note that this asm range is restartable i.e.
> EE is still on, so an interrupt (e.g. decrementer or external interrupt)
> can hit while SRR/GPRs are being loaded. This is defined via:
> 
> RESTART_TABLE(.Lsyscall_rst_start, .Lsyscall_rst_end, syscall_restart)
> 
> This restart table then sends us to syscall_restart rather than resuming
> in the middle of the RFI. The same stub is also used if irq_happened
> already has a pending bit (soft-masked irq that has not been replayed
> yet (PowerPC special case of local_irq_disable())).
> 
> Here is a bit of a flow of sequence of code to visualize:
>    syscall_exit_prepare
>        decide full-GPR restore (_TIF_RESTOREALL) for signal,
>        rt_sigreturn or syscall trace
>        save that in regs->exit_result and return it in r3
>             |
>             v
>    .Lsyscall_rst_start .. _end     EE still on
>        irq_happened set or interrupt in this range?
>             | no                         | yes
>             v                            v
>        cmpdi r3,0                  syscall_exit_restart
>        restore all / zero            replay irq, try exit again
>        volatiles; RFI                must return flags in r3
>                                      again for the same cmpdi
> 
> Now r3 after prepare is the flags word, not the actual syscall return. A nested
> interrupt clobbers it, so the restart stub reloads RESULT into r3 and the
> C handler (syscall_exit_restart()) should put the flags back (because later asm
> checks whether r3 returned from C has _TIF_RESTOREALL set or not):
> 	cmpdi r3, 0
> 	bne	.Lsyscall_restore_regs
> 
> Note that syscall_exit_restart() already ORs any new _TIF_RESTOREALL into
> exit_result, but then it only returns the new sample and not the full
> regs->exit_result.
> 
> That sample could be often 0 even when restore-all is still required:
> 
>    - rt_sigreturn / syscall trace set the bit in prepare's local
>      ret and in exit_result. They never set exit_flags, which is
>      what restart samples.
> 
>    - a signal does set exit_flags but restart clears it. A
>      second pass through the stub then returns 0 while
>      exit_result still has the bit.
> 
> The asm as mentioned earlier then treats r3==0 as the fast path and
> zeros r0/r4-r12. That means the userspace that needed the full register
> set could SIGSEGVs, (which could happen often in ld64.so.2 like while
> doing a parallel kernel build as reported by Venkat).
> 
> So we should instead return the accumulated exit_result, like how we do
> in interrupt_exit_user_restart(). Note that prior to this commit
> 263e5159e00a ("powerpc: Fix exit_flags field placement in pt_regs for ptrace")
> we were returning regs->exit_result from syscall_exit_restart(), but
> this commit changed that behaviour.
> 

Thanks for the fix. segfaults in ld64.so.2 are no longer seen with kernel
compilation.

Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>

> Fixes: 263e5159e00a ("powerpc: Fix exit_flags field placement in pt_regs for ptrace")
> Reported-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
> Closes: https://lore.kernel.org/all/75419f88-eab9-444b-bf97-28a9765819ad@linux.ibm.com/
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> ---
> Sorry about the long commit msg. It took sometime for me to fully understand
> that complex path, so I thought I may as well document that properly.
> 
>   arch/powerpc/kernel/interrupt.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
> index 5b88bf72786c..55f9c0c9922a 100644
> --- a/arch/powerpc/kernel/interrupt.c
> +++ b/arch/powerpc/kernel/interrupt.c
> @@ -175,7 +175,7 @@ notrace unsigned long syscall_exit_restart(unsigned long r3, struct pt_regs *reg
>   	current_thread_info()->exit_flags &= ~_TIF_RESTOREALL;
>   	regs->exit_result |= ret;
> 
> -	return ret;
> +	return regs->exit_result;
>   }
>   #endif
> 
> --
> 2.39.5
>