[PATCH] powerpc/syscall: Return the preset value of skipped syscalls
From: Renzo Davoli
Date: Sun Oct 18 2026 - 11:22:44 EST
From: Davide Berardi <berardi.dav@xxxxxxxxx>
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 <msuchanek@xxxxxxx>
Cc: Mukesh Kumar Chaurasiya (IBM) <mkchauras@xxxxxxxxx>
Cc: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxx>
Assisted-by: claude-opus-5-5
Signed-off-by: Renzo Davoli <renzo@xxxxxxxxxxx>
---
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
- 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