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

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

On 8/31/26 10:09 AM, Mukesh Kumar Chaurasiya wrote:
> 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@linux.ibm.com>
>> Closes: https://lore.kernel.org/all/e301014d-568f-4ed5-bc64-b8a85ca0b1e1@linux.ibm.com/
>> 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@linux.ibm.com>
>> ---
>> 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.
> 

I believe it is a pre-existing bug which can be fixed independent of this issue.
Since merge request broke the userspace it is better we fix it right away and
this pre-existing issue fix can be done subsequently.

No?

> 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
>>