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

["Michal Suchánek" <msuchanek@suse.de>]

Hello,

On Wed, Jun 24, 2026 at 10:45:20PM +0530, Mukesh Kumar Chaurasiya (IBM) wrote:
> 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>
> 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))
> +		return regs->gpr[3];

this should use syscall_get_error instead of accessing the register
value directly. The error is represented differently in the scv and
non-scv case, and for non-scv case this will return wrong value.

Thanks

Michal

> +
>  	if (unlikely(r0 >= NR_syscalls)) {
>  		if (unlikely(trap_is_unsupported_scv(regs))) {
>  			/* Unsupported scv vector */
> -- 
> 2.54.0
>