On Sun, Oct 18, 2026 at 05:15:33PM +0200, Renzo Davoli wrote:
> From: Davide Berardi <[email protected]>
>
> When ptrace or seccomp skip a system call, the return value they set in
> regs->gpr[3] gets lost or the tracer is affected, since two recent
> changes:
>
> 1. Commit 05c033db7e9a ("entry, treewide: Make
> syscall_enter_from_user_mode[_work]() indicate syscall execution")
> made system_call_exception() return syscall_get_error() for a skipped
> system call. syscall_get_error() is 0 for any non-error value, so a
> success value set by a tracer at a PTRACE_EVENT_SECCOMP stop (e.g.
> with PTRACE_POKEUSER), or by a SECCOMP_RET_USER_NOTIF listener, is
> replaced with 0.
>
> 2. Commit 69cb2be898be ("powerpc/syscall: Fix syscall skip handling for
> seccomp and ptrace") made syscall_set_return_value() set
> TIF_SYSCALL_RET with set_thread_flag(), i.e. on current rather than on
> the task passed in. When the caller is a tracer, as in
> PTRACE_SET_SYSCALL_INFO with op == PTRACE_SYSCALL_INFO_EXIT, the flag
> ends up on the tracer, and the next system call of the tracer is
> silently not executed. Setting it on the right task would not be
> enough either: at a syscall-exit stop the flag would then skip the
> next system call of the tracee.
>
> TIF_SYSCALL_RET is not needed anymore: after the change described in 1.
> the generic entry code tells whether ptrace or seccomp skipped the
> system call, and every in-kernel caller of syscall_set_return_value() on
> the syscall entry path (SECCOMP_RET_ERRNO, SECCOMP_RET_TRACE without a
> tracer, SECCOMP_RET_USER_NOTIF) also makes it return false. A user
> calling syscall(-1) without tracer or seccomp still gets -ENOSYS.
>
> Remove the flag and return regs->gpr[3] for a skipped system call, as
> done before the conversion to GENERIC_ENTRY. The syscall exit path
> already converts a negative error value to the "sc" ABI, and
> syscall_set_return_value() keeps CR0.SO in sync for positive error
> values.
>
> Fixes: 69cb2be898be ("powerpc/syscall: Fix syscall skip handling for seccomp
> and ptrace")
> Fixes: 05c033db7e9a ("entry, treewide: Make
> syscall_enter_from_user_mode[_work]() indicate syscall execution")
> Cc: Michal Suchánek <[email protected]>
> Cc: Mukesh Kumar Chaurasiya (IBM) <[email protected]>
> Cc: Shrikanth Hegde <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> Assisted-by: claude-opus-5-5
> Signed-off-by: Renzo Davoli <[email protected]>
> ---
> How this was found and tested: QEMU pseries (TCG, -cpu power9), ppc64le
> guest config, v7.3-rc6 (7b63ef2d55f2), with a small reproducer
> (ppc_skip_repro.c, available on request) which
>
Hey,
Can you share the reproducer?
Regards,
Mukesh
> - sets the getpid() return value at a syscall-exit stop with
> PTRACE_SET_SYSCALL_INFO, then makes a syscall in the tracer;
> - skips getpid() at a PTRACE_EVENT_SECCOMP stop with PTRACE_POKEUSER
> (r0 = -1, r3/CR0.SO = return value), with both "sc" and "scv 0".
>
> v7.3-rc6 with this patch
> tracer syscall after SET_SYSCALL_INFO returns 0 correct
> skipped syscall, success value, sc/scv returns 0 correct
> skipped syscall, error value, sc/scv correct correct
> selftests/ptrace set_syscall_info test FAIL pass
> selftests/seccomp/seccomp_bpf 11 failures all pass (*)
>
> failing in v7.3-rc6: user_notification_{basic,child_pid_ns,
> sibling_pid_ns,fault_recv,addfd,addfd_rlimit,sync,fifo,wait_killable},
> TRACE_syscall.seccomp.syscall_faked, plus the timing-only one below.
>
> (*) user_notification_wait_killable_after_reply (100000 notifications
> with a 1 ms itimer) can hit the 30 s harness timeout under emulation,
> with and without this patch.
>
> Build-tested with W=1: ppc64le guest config, ppc64_defconfig (BE),
> pmac32_defconfig.
>
> arch/powerpc/include/asm/syscall.h | 6 ------
> arch/powerpc/include/asm/thread_info.h | 1 -
> arch/powerpc/kernel/syscall.c | 15 ++++++++-------
> 3 files changed, 8 insertions(+), 14 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/syscall.h
> b/arch/powerpc/include/asm/syscall.h
> index 32be22b995cc..4b3c52ed6e9d 100644
> --- a/arch/powerpc/include/asm/syscall.h
> +++ b/arch/powerpc/include/asm/syscall.h
> @@ -98,12 +98,6 @@ static inline void syscall_set_return_value(struct
> task_struct *task,
> regs->gpr[3] = val;
> }
> }
> - /*
> - * Mark that a return value has been explicitly set by seccomp or
> - * ptrace so that system_call_exception() can skip the syscall
> - * unconditionally, even when the user requested syscall(-1).
> - */
> - set_thread_flag(TIF_SYSCALL_RET);
> }
>
> static inline void syscall_get_arguments(struct task_struct *task,
> diff --git a/arch/powerpc/include/asm/thread_info.h
> b/arch/powerpc/include/asm/thread_info.h
> index 1e069a2e7ce8..0487e94d3416 100644
> --- a/arch/powerpc/include/asm/thread_info.h
> +++ b/arch/powerpc/include/asm/thread_info.h
> @@ -120,7 +120,6 @@ void arch_setup_new_exec(void);
> #endif
> #define TIF_POLLING_NRFLAG 19 /* true if poll_idle() is polling
> TIF_NEED_RESCHED */
> #define TIF_32BIT 20 /* 32 bit binary */
> -#define TIF_SYSCALL_RET 21 /* syscall error value set */
>
> /* as above, but as bit values */
> #define _TIF_SYSCALL_TRACE (1<<TIF_SYSCALL_TRACE)
> diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> index fbefe1927b10..52ceff4ef41b 100644
> --- a/arch/powerpc/kernel/syscall.c
> +++ b/arch/powerpc/kernel/syscall.c
> @@ -18,13 +18,14 @@ 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)))
> {
> - 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);
> + /*
> + * If ptrace or seccomp decided to skip the syscall, regs->gpr[3] holds
> + * the return value they set (or the unmodified first argument, e.g.
> + * for PTRACE_SYSEMU). Return it as is, the exit path takes care of
> + * the error encoding for the "sc" ABI.
> + */
> + if (unlikely(!syscall_enter_from_user_mode_randomize_stack(regs, &r0)))
> + return regs->gpr[3];
>
> if (unlikely(r0 >= NR_syscalls)) {
> if (unlikely(trap_is_unsupported_scv(regs))) {
> --
> 2.55.0
>
