Hi Mukesh, thanks. I think we agree on the diagnosis: as your comment says, the return value has already been stored in regs, so the problem is only that system_call_exception() returns syscall_get_error(), which is 0 for any non-error value.
To Michal's point: there is no place setting the value incorrectly; r3 holds the right value, it is syscall_get_error() that cannot report a successful one. I like your variant: syscall_get_error() ?: regs->gpr[3] leaves the error path exactly as it is today and only changes the success path. I retested it with the ptrace set_syscall_info selftest and ppc_skip_repro.c (see https://lore.kernel.org/lkml/[email protected]/) and it behaves the same as my v2. I will send a v3 rebased on current mainline, split in two: 1/2 remove TIF_SYSCALL_RET (Fixes: 69cb2be898be), as suggested by Michal 2/2 return the preset value of a skipped syscall (Fixes: 05c033db7e9a) using your expression. Would you like a Suggested-by for the return expression? renzo On Thu, Oct 29, 2026 at 12:48:42PM +0530, Mukesh Kumar Chaurasiya wrote: > On Tue, Oct 27, 2026 at 11:19:42AM +0100, Michal Suchánek wrote: > > Hello, > > > > On Fri, Oct 23, 2026 at 08:07:32PM +0200, Renzo Davoli wrote: > > > 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]> > > > Cc: [email protected] # v7.3 > > > 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 > > > > > > - 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. > > > > > > Changes in v2: > > > - Rebased on current mainline (734bef548a96), which now contains > > > commit 66236558dfac ("powerpc/syscall: Avoid atomic > > > test_and_clear_thread_flag on fast path"). That commit folded the > > > TIF_SYSCALL_RET check into the skip condition, so the v1 hunk in > > > system_call_exception() no longer applied. The resulting code is the > > > same as with v1. > > > - Link to v1: > > > https://lore.kernel.org/lkml/[email protected] > > > > > > arch/powerpc/include/asm/syscall.h | 6 ------ > > > arch/powerpc/include/asm/thread_info.h | 1 - > > > arch/powerpc/kernel/syscall.c | 13 ++++++++----- > > > 3 files changed, 8 insertions(+), 12 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 8529827d697f..52ceff4ef41b 100644 > > > --- a/arch/powerpc/kernel/syscall.c > > > +++ b/arch/powerpc/kernel/syscall.c > > > @@ -18,11 +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) || > > > - test_thread_flag(TIF_SYSCALL_RET))) { > > > - 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))) { > > > > This does two things that should be addressed separately: > > > > - eliminating the TIF_SYSCALL_RET flag > > > > The previous refactor aimed at removing this flag. If the kernel still > > builds when it's removed removing the flag and the branch that tests it > > is merely dead code eliminaation unless there is something creative > > going on somewhere. > > > > - replacing syscall_get_error() with regs->gpr[3] > > > > This is a very dubious change. If this does fix something this is likely > > papering over some place that sets the return value incorrectly. > > Instead of this change the place where the incorrect setting of the > > return value happens should be fixed. > > > > Thanks > > > > Michal > > I agree with Michal here. > > what do you think about this diff > > 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 4df3f4b5bd56..f4f35314a901 100644 > --- a/arch/powerpc/include/asm/thread_info.h > +++ b/arch/powerpc/include/asm/thread_info.h > @@ -110,7 +110,6 @@ void arch_setup_new_exec(void); > #endif > #define TIF_POLLING_NRFLAG 22 /* true if poll_idle() is polling > TIF_NEED_RESCHED */ > #define TIF_32BIT 23 /* 32 bit binary */ > -#define TIF_SYSCALL_RET 24 /* syscall error value set */ > > /* as above, but as bit values */ > #define _TIF_POLLING_NRFLAG (1<<TIF_POLLING_NRFLAG) > diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c > index fbefe1927b10..ec81046e9eb5 100644 > --- a/arch/powerpc/kernel/syscall.c > +++ b/arch/powerpc/kernel/syscall.c > @@ -19,13 +19,17 @@ notrace long system_call_exception(struct pt_regs *regs, > unsigned long r0) > 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); > + /* > + * The syscall was skipped by seccomp or ptrace, which have > + * already stored the result in the user ABI form via > + * syscall_set_return_value(). Hand it back in the form > + * syscall_exit_prepare() expects: -errno on failure, else the > + * value in r3. Using syscall_get_error() alone would drop a > + * successful value, e.g. from SECCOMP_USER_NOTIF. > + */ > + return syscall_get_error(current, regs) ?: regs->gpr[3]; > } > > - if (unlikely(test_and_clear_thread_flag(TIF_SYSCALL_RET))) > - return syscall_get_error(current, regs); > - > if (unlikely(r0 >= NR_syscalls)) { > if (unlikely(trap_is_unsupported_scv(regs))) { > /* Unsupported scv vector */ > > > > Regards, > Mukesh
