Re: [PATCH] powerpc/syscall: Fix seccomp errno handling with GENERIC_ENTRY

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

Le 24/06/2026 à 19:15, Mukesh Kumar Chaurasiya (IBM) a écrit :
> After enabling GENERIC_ENTRY on PowerPC, seccomp filters using
> SCMP_ACT_ERRNO without an explicit errnoRet value return ENOSYS
> (Function not implemented) instead of the expected EPERM (Operation
> not permitted).
> 
> The issue occurs in system_call_exception() when syscall_enter_from_user_mode()
> returns -1 to indicate the syscall should be skipped (e.g., blocked by seccomp).
> The current code treats this -1 as a syscall number and compares it against
> NR_syscalls. Since -1 (when cast to unsigned long) is greater than NR_syscalls,
> the code incorrectly returns -ENOSYS, overwriting the errno that seccomp
> already set via syscall_set_return_value().
> 
> The generic entry code in syscall_trace_enter() calls __secure_computing(),
> which sets the appropriate errno in regs->gpr[3] and returns -1 to signal
> that the syscall should be skipped. However, the PowerPC syscall handler
> was not checking for this -1 return value before validating the syscall
> number.
> 
> Fix this by explicitly checking if syscall_enter_from_user_mode() returns
> -1 and returning the value already set in regs->gpr[3] (the errno from
> seccomp) before performing the syscall number validation.
> 
> This aligns PowerPC's behavior with other architectures using GENERIC_ENTRY
> and restores correct seccomp errno handling.
> 
> Fixes: bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature")
> Reported-by: Michal Suchánek <msuchanek@suse.de>

Closes: https://lore.kernel.org/all/ajpp-_XnbF3UTM_E@kunlun.suse.cz/

> Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
> ---
>   arch/powerpc/kernel/syscall.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> index a9da2af6efa8..5b58c8d396c8 100644
> --- a/arch/powerpc/kernel/syscall.c
> +++ b/arch/powerpc/kernel/syscall.c
> @@ -22,6 +22,10 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
>   	add_random_kstack_offset();
>   	r0 = syscall_enter_from_user_mode(regs, r0);
>   
> +	/* Seccomp or ptrace may have set return value, skip syscall */
> +	if (unlikely(r0 == -1L))

Is it really needed to add the L after 1 ?

> +		return regs->gpr[3];
> +
>   	if (unlikely(r0 >= NR_syscalls)) {
>   		if (unlikely(trap_is_unsupported_scv(regs))) {
>   			/* Unsupported scv vector */