Re: [PATCH] powerpc/entry: Clear TIF_SYSCALL_RET before syscall error return

[Mukesh Kumar Chaurasiya]

On Fri, Aug 28, 2026 at 11:08:11AM +0530, Shrikanth Hegde wrote:
> Shivaprasad reported a boot failure due to userspace processes crash on
> abort() from libc.so.6. It was bisected to merge request
> commit '3424d8c18a7d ("Merge tag  'core-entry-2026-08-17' of 
> git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip")'
> 
> Upon checking the merge, when syscall_enter_from_user_mode_randomize_stack
> fails, which could happen when a tracer like seccomp or ptrace intercepts
> and skips the syscall, the code returns to userspace immediately without
> clearing the intermediate flag which was set.
> 
> When the next syscall is made, it immediately aborts the valid syscall
> since the flag is still set. Hence clear the flag on occurrence of first
> failure.
> 
> Reported-by: Shivaprasad G Bhat <sbhat@xxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/all/e301014d-568f-4ed5-bc64-b8a85ca0b1e1@xxxxxxxxxxxxx/
> Fixes: 3424d8c18a7d ("Merge tag 'core-entry-2026-08-17' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip")
> Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> ---
> PS: I have kept the block below since earlier code was checking it
> regardless of result of syscall_enter_from_user_mode. If it turns out
> to be a redundant, it can be removed later.
> 
I think it is redundant as of now. The TIF_SYSCALL_RET flag is set when
error is set by the ptrace or seccomp, If the error value is set then
the syscall_enter_from_user_mode_randomize_stack will return false.
Hence the next check will become redundant. 

I also see that TIF_SYSCALL_RET is also set when processing the ptrace
syscall, Which can leave the flag set for next syscall execution. Which
can again trigger the same issue.

Regards,
Mukesh
>  arch/powerpc/kernel/syscall.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> index 4916c205c4bb..fbefe1927b10 100644
> --- a/arch/powerpc/kernel/syscall.c
> +++ b/arch/powerpc/kernel/syscall.c
> @@ -18,8 +18,10 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
>  	long ret;
>  	syscall_fn f;
>  
> -	if (unlikely(!syscall_enter_from_user_mode_randomize_stack(regs, &r0)))
> +	if (unlikely(!syscall_enter_from_user_mode_randomize_stack(regs, &r0))) {
> +		clear_thread_flag(TIF_SYSCALL_RET);
>  		return syscall_get_error(current, regs);
> +	}
>  
>  	if (unlikely(test_and_clear_thread_flag(TIF_SYSCALL_RET)))
>  		return syscall_get_error(current, regs);
> -- 
> 2.47.3
>