[RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR

[RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR

[Michal Suchánek]

The return value of syscall_enter_from_user_mode is used both for the
adjusted syscall number and the indicator that a syscall should be
skipped.

As seccomp can be invoked on any syscall, including invalid ones this
somewhat undermines seccomp.

While the seccomp variants that terminate the process do not need to
care about this for the filter that sets the syscall return value this
disctinction is required.

Pass the syscall number as a pointer to the inline entry functions, and
use the return value exclusively for the indication that the syscall is
already handled.

This should avoid the need for the s390 PIF_SYSCALL_RET_SET which is the
workaround for exactly this deficiency.

If this is desirable the patch could be split into some series that
adjusts the code flow where needed so that the final change is mostly
mechanical.

There is also another way to handle this problem.

With x86 using bit 30 to denote compatibility syscall it sounds like
declaring syscall number a 30bit quantity would work.

Then bit 31 could be used to denote an invalid syscall that can never be
executed, and the -1 returned from syscall_enter_from_user_mode would
then be inherently invalid.

That is so long as no architectures use syscall numbers outside of this
range so far, and the limitation is considered fine.

Signed-off-by: Michal Suchánek <msuchanek@suse.de>
---
 Documentation/core-api/entry.rst | 12 ++++++----
 arch/loongarch/kernel/syscall.c  |  6 ++---
 arch/powerpc/kernel/syscall.c    |  3 ++-
 arch/riscv/kernel/traps.c        |  6 ++---
 arch/s390/kernel/syscall.c       |  6 ++---
 arch/x86/entry/syscall_32.c      | 39 +++++++++++++++-----------------
 arch/x86/entry/syscall_64.c      | 19 ++++++++--------
 include/linux/entry-common.h     | 38 ++++++++++++++-----------------
 8 files changed, 63 insertions(+), 66 deletions(-)

diff --git a/Documentation/core-api/entry.rst b/Documentation/core-api/entry.rst
index 71d8eedc0549..b0bfae31fe7c 100644
--- a/Documentation/core-api/entry.rst
+++ b/Documentation/core-api/entry.rst
@@ -68,12 +68,14 @@ invoked from low-level assembly code looks like this:
   noinstr void syscall(struct pt_regs *regs, int nr)
   {
 	arch_syscall_enter(regs);
-	nr = syscall_enter_from_user_mode(regs, nr);
 
-	instrumentation_begin();
-	if (!invoke_syscall(regs, nr) && nr != -1)
-	 	result_reg(regs) = __sys_ni_syscall(regs);
-	instrumentation_end();
+	/* Skip syscall when -1 is returned */
+	if (!syscall_enter_from_user_mode(regs, &nr)) {
+		instrumentation_begin();
+		if (!invoke_syscall(regs, nr) && nr != -1)
+			result_reg(regs) = __sys_ni_syscall(regs);
+		instrumentation_end();
+	}
 
 	syscall_exit_to_user_mode(regs);
   }
diff --git a/arch/loongarch/kernel/syscall.c b/arch/loongarch/kernel/syscall.c
index 94c1c3b5b0b5..fc18ac56b91c 100644
--- a/arch/loongarch/kernel/syscall.c
+++ b/arch/loongarch/kernel/syscall.c
@@ -58,7 +58,7 @@ typedef long (*sys_call_fn)(unsigned long, unsigned long,
 
 void noinstr __no_stack_protector do_syscall(struct pt_regs *regs)
 {
-	unsigned long nr;
+	unsigned long nr, ret;
 	sys_call_fn syscall_fn;
 
 	nr = regs->regs[11];
@@ -70,11 +70,11 @@ void noinstr __no_stack_protector do_syscall(struct pt_regs *regs)
 	regs->orig_a0 = regs->regs[4];
 	regs->regs[4] = -ENOSYS;
 
-	nr = syscall_enter_from_user_mode(regs, nr);
+	ret = syscall_enter_from_user_mode(regs, &nr);
 
 	add_random_kstack_offset();
 
-	if (nr < NR_syscalls) {
+	if (nr < NR_syscalls && !ret) {
 		syscall_fn = sys_call_table[array_index_nospec(nr, NR_syscalls)];
 		regs->regs[4] = syscall_fn(regs->orig_a0, regs->regs[5], regs->regs[6],
 					   regs->regs[7], regs->regs[8], regs->regs[9]);
diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
index a9da2af6efa8..45d11d518c51 100644
--- a/arch/powerpc/kernel/syscall.c
+++ b/arch/powerpc/kernel/syscall.c
@@ -20,7 +20,8 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
 	syscall_fn f;
 
 	add_random_kstack_offset();
-	r0 = syscall_enter_from_user_mode(regs, r0);
+	if (unlikely(syscall_enter_from_user_mode(regs, &r0)))
+		return syscall_get_error(current, regs);
 
 	if (unlikely(r0 >= NR_syscalls)) {
 		if (unlikely(trap_is_unsupported_scv(regs))) {
diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index 8c62c771a656..9326a4a50696 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -325,7 +325,7 @@ asmlinkage __visible __trap_section  __no_stack_protector
 void do_trap_ecall_u(struct pt_regs *regs)
 {
 	if (user_mode(regs)) {
-		long syscall = regs->a7;
+		long ret, syscall = regs->a7;
 
 		regs->epc += 4;
 		regs->orig_a0 = regs->a0;
@@ -333,11 +333,11 @@ void do_trap_ecall_u(struct pt_regs *regs)
 
 		riscv_v_vstate_discard(regs);
 
-		syscall = syscall_enter_from_user_mode(regs, syscall);
+		ret = syscall_enter_from_user_mode(regs, &syscall);
 
 		add_random_kstack_offset();
 
-		if (syscall >= 0 && syscall < NR_syscalls) {
+		if (syscall >= 0 && syscall < NR_syscalls && !ret) {
 			syscall = array_index_nospec(syscall, NR_syscalls);
 			syscall_handler(regs, syscall);
 		}
diff --git a/arch/s390/kernel/syscall.c b/arch/s390/kernel/syscall.c
index 75d5a3cab14e..9e5b873c011d 100644
--- a/arch/s390/kernel/syscall.c
+++ b/arch/s390/kernel/syscall.c
@@ -95,7 +95,7 @@ SYSCALL_DEFINE0(ni_syscall)
 
 void noinstr __do_syscall(struct pt_regs *regs, int per_trap)
 {
-	unsigned long nr;
+	unsigned long nr, ret;
 
 	enter_from_user_mode(regs);
 	add_random_kstack_offset();
@@ -121,7 +121,7 @@ void noinstr __do_syscall(struct pt_regs *regs, int per_trap)
 		regs->psw.addr = current->restart_block.arch_data;
 		current->restart_block.arch_data = 1;
 	}
-	nr = syscall_enter_from_user_mode_work(regs, nr);
+	ret = syscall_enter_from_user_mode_work(regs, &nr);
 	/*
 	 * In the s390 ptrace ABI, both the syscall number and the return value
 	 * use gpr2. However, userspace puts the syscall number either in the
@@ -129,7 +129,7 @@ void noinstr __do_syscall(struct pt_regs *regs, int per_trap)
 	 * work, the ptrace code sets PIF_SYSCALL_RET_SET, which is checked here
 	 * and if set, the syscall will be skipped.
 	 */
-	if (unlikely(test_and_clear_pt_regs_flag(regs, PIF_SYSCALL_RET_SET)))
+	if (unlikely(test_and_clear_pt_regs_flag(regs, PIF_SYSCALL_RET_SET) || ret))
 		goto out;
 	regs->gprs[2] = -ENOSYS;
 	if (likely(nr < NR_syscalls)) {
diff --git a/arch/x86/entry/syscall_32.c b/arch/x86/entry/syscall_32.c
index 31b9492fe851..525e99691b31 100644
--- a/arch/x86/entry/syscall_32.c
+++ b/arch/x86/entry/syscall_32.c
@@ -128,7 +128,7 @@ static __always_inline bool int80_is_external(void)
  */
 __visible noinstr void do_int80_emulation(struct pt_regs *regs)
 {
-	int nr;
+	long nr;
 
 	/* Kernel does not use INT $0x80! */
 	if (unlikely(!user_mode(regs))) {
@@ -168,8 +168,7 @@ __visible noinstr void do_int80_emulation(struct pt_regs *regs)
 	nr = syscall_32_enter(regs);
 
 	local_irq_enable();
-	nr = syscall_enter_from_user_mode_work(regs, nr);
-	do_syscall_32_irqs_on(regs, nr);
+	syscall_enter_from_user_mode_work(regs, &nr);
 
 	instrumentation_end();
 	syscall_exit_to_user_mode(regs);
@@ -208,7 +207,7 @@ __visible noinstr void do_int80_emulation(struct pt_regs *regs)
  */
 DEFINE_FREDENTRY_RAW(int80_emulation)
 {
-	int nr;
+	long nr;
 
 	enter_from_user_mode(regs);
 
@@ -232,8 +231,10 @@ DEFINE_FREDENTRY_RAW(int80_emulation)
 	nr = syscall_32_enter(regs);
 
 	local_irq_enable();
-	nr = syscall_enter_from_user_mode_work(regs, nr);
-	do_syscall_32_irqs_on(regs, nr);
+	if (!syscall_enter_from_user_mode_work(regs, &nr)) {
+		nr &= GENMASK(31, 0);
+		do_syscall_32_irqs_on(regs, nr);
+	}
 
 	instrumentation_end();
 	syscall_exit_to_user_mode(regs);
@@ -245,20 +246,17 @@ DEFINE_FREDENTRY_RAW(int80_emulation)
 /* Handles int $0x80 on a 32bit kernel */
 __visible noinstr void do_int80_syscall_32(struct pt_regs *regs)
 {
-	int nr = syscall_32_enter(regs);
-
-	/*
-	 * Subtlety here: if ptrace pokes something larger than 2^31-1 into
-	 * orig_ax, the int return value truncates it. This matches
-	 * the semantics of syscall_get_nr().
-	 */
-	nr = syscall_enter_from_user_mode(regs, nr);
-	instrumentation_begin();
+	long nr = syscall_32_enter(regs);
 
 	add_random_kstack_offset();
-	do_syscall_32_irqs_on(regs, nr);
+	if (!syscall_enter_from_user_mode(regs, &nr)) {
+		instrumentation_begin();
 
-	instrumentation_end();
+		nr &= & GENMASK(31, 0);
+		do_syscall_32_irqs_on(regs, nr);
+
+		instrumentation_end();
+	}
 	syscall_exit_to_user_mode(regs);
 }
 #endif /* !CONFIG_IA32_EMULATION */
@@ -301,10 +299,9 @@ static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)
 		return false;
 	}
 
-	nr = syscall_enter_from_user_mode_work(regs, nr);
-
-	/* Now this is just like a normal syscall. */
-	do_syscall_32_irqs_on(regs, nr);
+	if (!syscall_enter_from_user_mode_work(regs, &nr))
+		/* Now this is just like a normal syscall. */
+		do_syscall_32_irqs_on(regs, nr);
 
 	instrumentation_end();
 	syscall_exit_to_user_mode(regs);
diff --git a/arch/x86/entry/syscall_64.c b/arch/x86/entry/syscall_64.c
index 71f032504e73..3400c2f43a62 100644
--- a/arch/x86/entry/syscall_64.c
+++ b/arch/x86/entry/syscall_64.c
@@ -84,19 +84,20 @@ static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr)
 }
 
 /* Returns true to return using SYSRET, or false to use IRET */
-__visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
+__visible noinstr bool do_syscall_64(struct pt_regs *regs, long nr)
 {
-	nr = syscall_enter_from_user_mode(regs, nr);
-
-	instrumentation_begin();
 	add_random_kstack_offset();
+	if (!syscall_enter_from_user_mode(regs, &nr)) {
 
-	if (!do_syscall_x64(regs, nr) && !do_syscall_x32(regs, nr) && nr != -1) {
-		/* Invalid system call, but still a system call. */
-		regs->ax = __x64_sys_ni_syscall(regs);
-	}
+		instrumentation_begin();
 
-	instrumentation_end();
+		if (!do_syscall_x64(regs, nr) && !do_syscall_x32(regs, nr)) {
+			/* Invalid system call, but still a system call. */
+			regs->ax = __x64_sys_ni_syscall(regs);
+		}
+
+		instrumentation_end();
+	}
 	syscall_exit_to_user_mode(regs);
 
 	/*
diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index 416a3352261f..4991071d01fe 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -69,10 +69,8 @@ static inline void syscall_enter_audit(struct pt_regs *regs, long syscall)
 	}
 }
 
-static __always_inline long syscall_trace_enter(struct pt_regs *regs, unsigned long work)
+static __always_inline long syscall_trace_enter(struct pt_regs *regs, unsigned long work, unsigned long *syscall)
 {
-	long syscall, ret = 0;
-
 	/*
 	 * Handle Syscall User Dispatch.  This must comes first, since
 	 * the ABI here can be something that doesn't make sense for
@@ -93,27 +91,25 @@ static __always_inline long syscall_trace_enter(struct pt_regs *regs, unsigned l
 
 	/* Handle ptrace */
 	if (work & (SYSCALL_WORK_SYSCALL_TRACE | SYSCALL_WORK_SYSCALL_EMU)) {
-		ret = arch_ptrace_report_syscall_entry(regs);
-		if (ret || (work & SYSCALL_WORK_SYSCALL_EMU))
+		if (arch_ptrace_report_syscall_entry(regs) || (work & SYSCALL_WORK_SYSCALL_EMU))
 			return -1L;
 	}
 
 	/* Do seccomp after ptrace, to catch any tracer changes. */
 	if (work & SYSCALL_WORK_SECCOMP) {
-		ret = __secure_computing();
-		if (ret == -1L)
-			return ret;
+		if (__secure_computing())
+			return -1L;
 	}
 
 	/* Either of the above might have changed the syscall number */
-	syscall = syscall_get_nr(current, regs);
+	*syscall = syscall_get_nr(current, regs);
 
 	if (unlikely(work & SYSCALL_WORK_SYSCALL_TRACEPOINT))
-		syscall = trace_syscall_enter(regs, syscall);
+		*syscall = trace_syscall_enter(regs, *syscall);
 
-	syscall_enter_audit(regs, syscall);
+	syscall_enter_audit(regs, *syscall);
 
-	return ret ? : syscall;
+	return 0;
 }
 
 /**
@@ -126,12 +122,12 @@ static __always_inline long syscall_trace_enter(struct pt_regs *regs, unsigned l
  * enabled after invoking enter_from_user_mode(), enabling interrupts and
  * extra architecture specific work.
  *
- * Returns: The original or a modified syscall number
+ * Returns: The original or a modified syscall number as syscall
  *
- * If the returned syscall number is -1 then the syscall should be
- * skipped. In this case the caller may invoke syscall_set_error() or
- * syscall_set_return_value() first.  If neither of those are called and -1
- * is returned, then the syscall will fail with ENOSYS.
+ * If the returned value is -1 then the syscall should be skipped. In this case
+ * the caller may invoke syscall_set_error() or syscall_set_return_value()
+ * first.  If neither of those are called and -1 is returned, then the syscall
+ * will fail with ENOSYS.
  *
  * It handles the following work items:
  *
@@ -139,14 +135,14 @@ static __always_inline long syscall_trace_enter(struct pt_regs *regs, unsigned l
  *     ptrace_report_syscall_entry(), __secure_computing(), trace_sys_enter()
  *  2) Invocation of audit_syscall_entry()
  */
-static __always_inline long syscall_enter_from_user_mode_work(struct pt_regs *regs, long syscall)
+static __always_inline long syscall_enter_from_user_mode_work(struct pt_regs *regs, long *syscall)
 {
 	unsigned long work = READ_ONCE(current_thread_info()->syscall_work);
 
 	if (work & SYSCALL_WORK_ENTER)
-		syscall = syscall_trace_enter(regs, work);
+		return syscall_trace_enter(regs, work, syscall);
 
-	return syscall;
+	return 0;
 }
 
 /**
@@ -167,7 +163,7 @@ static __always_inline long syscall_enter_from_user_mode_work(struct pt_regs *re
  * Returns: The original or a modified syscall number. See
  * syscall_enter_from_user_mode_work() for further explanation.
  */
-static __always_inline long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall)
+static __always_inline long syscall_enter_from_user_mode(struct pt_regs *regs, long *syscall)
 {
 	long ret;
 
-- 
2.51.0

Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR

[H. Peter Anvin]

On July 1, 2026 10:42:08 AM PDT, "Michal Suchánek" <msuchanek@suse.de> wrote:
>The return value of syscall_enter_from_user_mode is used both for the
>adjusted syscall number and the indicator that a syscall should be
>skipped.
>
>As seccomp can be invoked on any syscall, including invalid ones this
>somewhat undermines seccomp.
>
>While the seccomp variants that terminate the process do not need to
>care about this for the filter that sets the syscall return value this
>disctinction is required.
>
>Pass the syscall number as a pointer to the inline entry functions, and
>use the return value exclusively for the indication that the syscall is
>already handled.
>
>This should avoid the need for the s390 PIF_SYSCALL_RET_SET which is the
>workaround for exactly this deficiency.
>
>If this is desirable the patch could be split into some series that
>adjusts the code flow where needed so that the final change is mostly
>mechanical.
>
>There is also another way to handle this problem.
>
>With x86 using bit 30 to denote compatibility syscall it sounds like
>declaring syscall number a 30bit quantity would work.
>
>Then bit 31 could be used to denote an invalid syscall that can never be
>executed, and the -1 returned from syscall_enter_from_user_mode would
>then be inherently invalid.
>
>That is so long as no architectures use syscall numbers outside of this
>range so far, and the limitation is considered fine.
>
>Signed-off-by: Michal Suchánek <msuchanek@suse.de>
>---
> Documentation/core-api/entry.rst | 12 ++++++----
> arch/loongarch/kernel/syscall.c  |  6 ++---
> arch/powerpc/kernel/syscall.c    |  3 ++-
> arch/riscv/kernel/traps.c        |  6 ++---
> arch/s390/kernel/syscall.c       |  6 ++---
> arch/x86/entry/syscall_32.c      | 39 +++++++++++++++-----------------
> arch/x86/entry/syscall_64.c      | 19 ++++++++--------
> include/linux/entry-common.h     | 38 ++++++++++++++-----------------
> 8 files changed, 63 insertions(+), 66 deletions(-)
>
>diff --git a/Documentation/core-api/entry.rst b/Documentation/core-api/entry.rst
>index 71d8eedc0549..b0bfae31fe7c 100644
>--- a/Documentation/core-api/entry.rst
>+++ b/Documentation/core-api/entry.rst
>@@ -68,12 +68,14 @@ invoked from low-level assembly code looks like this:
>   noinstr void syscall(struct pt_regs *regs, int nr)
>   {
> 	arch_syscall_enter(regs);
>-	nr = syscall_enter_from_user_mode(regs, nr);
> 
>-	instrumentation_begin();
>-	if (!invoke_syscall(regs, nr) && nr != -1)
>-	 	result_reg(regs) = __sys_ni_syscall(regs);
>-	instrumentation_end();
>+	/* Skip syscall when -1 is returned */
>+	if (!syscall_enter_from_user_mode(regs, &nr)) {
>+		instrumentation_begin();
>+		if (!invoke_syscall(regs, nr) && nr != -1)
>+			result_reg(regs) = __sys_ni_syscall(regs);
>+		instrumentation_end();
>+	}
> 
> 	syscall_exit_to_user_mode(regs);
>   }
>diff --git a/arch/loongarch/kernel/syscall.c b/arch/loongarch/kernel/syscall.c
>index 94c1c3b5b0b5..fc18ac56b91c 100644
>--- a/arch/loongarch/kernel/syscall.c
>+++ b/arch/loongarch/kernel/syscall.c
>@@ -58,7 +58,7 @@ typedef long (*sys_call_fn)(unsigned long, unsigned long,
> 
> void noinstr __no_stack_protector do_syscall(struct pt_regs *regs)
> {
>-	unsigned long nr;
>+	unsigned long nr, ret;
> 	sys_call_fn syscall_fn;
> 
> 	nr = regs->regs[11];
>@@ -70,11 +70,11 @@ void noinstr __no_stack_protector do_syscall(struct pt_regs *regs)
> 	regs->orig_a0 = regs->regs[4];
> 	regs->regs[4] = -ENOSYS;
> 
>-	nr = syscall_enter_from_user_mode(regs, nr);
>+	ret = syscall_enter_from_user_mode(regs, &nr);
> 
> 	add_random_kstack_offset();
> 
>-	if (nr < NR_syscalls) {
>+	if (nr < NR_syscalls && !ret) {
> 		syscall_fn = sys_call_table[array_index_nospec(nr, NR_syscalls)];
> 		regs->regs[4] = syscall_fn(regs->orig_a0, regs->regs[5], regs->regs[6],
> 					   regs->regs[7], regs->regs[8], regs->regs[9]);
>diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
>index a9da2af6efa8..45d11d518c51 100644
>--- a/arch/powerpc/kernel/syscall.c
>+++ b/arch/powerpc/kernel/syscall.c
>@@ -20,7 +20,8 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
> 	syscall_fn f;
> 
> 	add_random_kstack_offset();
>-	r0 = syscall_enter_from_user_mode(regs, r0);
>+	if (unlikely(syscall_enter_from_user_mode(regs, &r0)))
>+		return syscall_get_error(current, regs);
> 
> 	if (unlikely(r0 >= NR_syscalls)) {
> 		if (unlikely(trap_is_unsupported_scv(regs))) {
>diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
>index 8c62c771a656..9326a4a50696 100644
>--- a/arch/riscv/kernel/traps.c
>+++ b/arch/riscv/kernel/traps.c
>@@ -325,7 +325,7 @@ asmlinkage __visible __trap_section  __no_stack_protector
> void do_trap_ecall_u(struct pt_regs *regs)
> {
> 	if (user_mode(regs)) {
>-		long syscall = regs->a7;
>+		long ret, syscall = regs->a7;
> 
> 		regs->epc += 4;
> 		regs->orig_a0 = regs->a0;
>@@ -333,11 +333,11 @@ void do_trap_ecall_u(struct pt_regs *regs)
> 
> 		riscv_v_vstate_discard(regs);
> 
>-		syscall = syscall_enter_from_user_mode(regs, syscall);
>+		ret = syscall_enter_from_user_mode(regs, &syscall);
> 
> 		add_random_kstack_offset();
> 
>-		if (syscall >= 0 && syscall < NR_syscalls) {
>+		if (syscall >= 0 && syscall < NR_syscalls && !ret) {
> 			syscall = array_index_nospec(syscall, NR_syscalls);
> 			syscall_handler(regs, syscall);
> 		}
>diff --git a/arch/s390/kernel/syscall.c b/arch/s390/kernel/syscall.c
>index 75d5a3cab14e..9e5b873c011d 100644
>--- a/arch/s390/kernel/syscall.c
>+++ b/arch/s390/kernel/syscall.c
>@@ -95,7 +95,7 @@ SYSCALL_DEFINE0(ni_syscall)
> 
> void noinstr __do_syscall(struct pt_regs *regs, int per_trap)
> {
>-	unsigned long nr;
>+	unsigned long nr, ret;
> 
> 	enter_from_user_mode(regs);
> 	add_random_kstack_offset();
>@@ -121,7 +121,7 @@ void noinstr __do_syscall(struct pt_regs *regs, int per_trap)
> 		regs->psw.addr = current->restart_block.arch_data;
> 		current->restart_block.arch_data = 1;
> 	}
>-	nr = syscall_enter_from_user_mode_work(regs, nr);
>+	ret = syscall_enter_from_user_mode_work(regs, &nr);
> 	/*
> 	 * In the s390 ptrace ABI, both the syscall number and the return value
> 	 * use gpr2. However, userspace puts the syscall number either in the
>@@ -129,7 +129,7 @@ void noinstr __do_syscall(struct pt_regs *regs, int per_trap)
> 	 * work, the ptrace code sets PIF_SYSCALL_RET_SET, which is checked here
> 	 * and if set, the syscall will be skipped.
> 	 */
>-	if (unlikely(test_and_clear_pt_regs_flag(regs, PIF_SYSCALL_RET_SET)))
>+	if (unlikely(test_and_clear_pt_regs_flag(regs, PIF_SYSCALL_RET_SET) || ret))
> 		goto out;
> 	regs->gprs[2] = -ENOSYS;
> 	if (likely(nr < NR_syscalls)) {
>diff --git a/arch/x86/entry/syscall_32.c b/arch/x86/entry/syscall_32.c
>index 31b9492fe851..525e99691b31 100644
>--- a/arch/x86/entry/syscall_32.c
>+++ b/arch/x86/entry/syscall_32.c
>@@ -128,7 +128,7 @@ static __always_inline bool int80_is_external(void)
>  */
> __visible noinstr void do_int80_emulation(struct pt_regs *regs)
> {
>-	int nr;
>+	long nr;
> 
> 	/* Kernel does not use INT $0x80! */
> 	if (unlikely(!user_mode(regs))) {
>@@ -168,8 +168,7 @@ __visible noinstr void do_int80_emulation(struct pt_regs *regs)
> 	nr = syscall_32_enter(regs);
> 
> 	local_irq_enable();
>-	nr = syscall_enter_from_user_mode_work(regs, nr);
>-	do_syscall_32_irqs_on(regs, nr);
>+	syscall_enter_from_user_mode_work(regs, &nr);
> 
> 	instrumentation_end();
> 	syscall_exit_to_user_mode(regs);
>@@ -208,7 +207,7 @@ __visible noinstr void do_int80_emulation(struct pt_regs *regs)
>  */
> DEFINE_FREDENTRY_RAW(int80_emulation)
> {
>-	int nr;
>+	long nr;
> 
> 	enter_from_user_mode(regs);
> 
>@@ -232,8 +231,10 @@ DEFINE_FREDENTRY_RAW(int80_emulation)
> 	nr = syscall_32_enter(regs);
> 
> 	local_irq_enable();
>-	nr = syscall_enter_from_user_mode_work(regs, nr);
>-	do_syscall_32_irqs_on(regs, nr);
>+	if (!syscall_enter_from_user_mode_work(regs, &nr)) {
>+		nr &= GENMASK(31, 0);
>+		do_syscall_32_irqs_on(regs, nr);
>+	}
> 
> 	instrumentation_end();
> 	syscall_exit_to_user_mode(regs);
>@@ -245,20 +246,17 @@ DEFINE_FREDENTRY_RAW(int80_emulation)
> /* Handles int $0x80 on a 32bit kernel */
> __visible noinstr void do_int80_syscall_32(struct pt_regs *regs)
> {
>-	int nr = syscall_32_enter(regs);
>-
>-	/*
>-	 * Subtlety here: if ptrace pokes something larger than 2^31-1 into
>-	 * orig_ax, the int return value truncates it. This matches
>-	 * the semantics of syscall_get_nr().
>-	 */
>-	nr = syscall_enter_from_user_mode(regs, nr);
>-	instrumentation_begin();
>+	long nr = syscall_32_enter(regs);
> 
> 	add_random_kstack_offset();
>-	do_syscall_32_irqs_on(regs, nr);
>+	if (!syscall_enter_from_user_mode(regs, &nr)) {
>+		instrumentation_begin();
> 
>-	instrumentation_end();
>+		nr &= & GENMASK(31, 0);
>+		do_syscall_32_irqs_on(regs, nr);
>+
>+		instrumentation_end();
>+	}
> 	syscall_exit_to_user_mode(regs);
> }
> #endif /* !CONFIG_IA32_EMULATION */
>@@ -301,10 +299,9 @@ static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)
> 		return false;
> 	}
> 
>-	nr = syscall_enter_from_user_mode_work(regs, nr);
>-
>-	/* Now this is just like a normal syscall. */
>-	do_syscall_32_irqs_on(regs, nr);
>+	if (!syscall_enter_from_user_mode_work(regs, &nr))
>+		/* Now this is just like a normal syscall. */
>+		do_syscall_32_irqs_on(regs, nr);
> 
> 	instrumentation_end();
> 	syscall_exit_to_user_mode(regs);
>diff --git a/arch/x86/entry/syscall_64.c b/arch/x86/entry/syscall_64.c
>index 71f032504e73..3400c2f43a62 100644
>--- a/arch/x86/entry/syscall_64.c
>+++ b/arch/x86/entry/syscall_64.c
>@@ -84,19 +84,20 @@ static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr)
> }
> 
> /* Returns true to return using SYSRET, or false to use IRET */
>-__visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
>+__visible noinstr bool do_syscall_64(struct pt_regs *regs, long nr)
> {
>-	nr = syscall_enter_from_user_mode(regs, nr);
>-
>-	instrumentation_begin();
> 	add_random_kstack_offset();
>+	if (!syscall_enter_from_user_mode(regs, &nr)) {
> 
>-	if (!do_syscall_x64(regs, nr) && !do_syscall_x32(regs, nr) && nr != -1) {
>-		/* Invalid system call, but still a system call. */
>-		regs->ax = __x64_sys_ni_syscall(regs);
>-	}
>+		instrumentation_begin();
> 
>-	instrumentation_end();
>+		if (!do_syscall_x64(regs, nr) && !do_syscall_x32(regs, nr)) {
>+			/* Invalid system call, but still a system call. */
>+			regs->ax = __x64_sys_ni_syscall(regs);
>+		}
>+
>+		instrumentation_end();
>+	}
> 	syscall_exit_to_user_mode(regs);
> 
> 	/*
>diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
>index 416a3352261f..4991071d01fe 100644
>--- a/include/linux/entry-common.h
>+++ b/include/linux/entry-common.h
>@@ -69,10 +69,8 @@ static inline void syscall_enter_audit(struct pt_regs *regs, long syscall)
> 	}
> }
> 
>-static __always_inline long syscall_trace_enter(struct pt_regs *regs, unsigned long work)
>+static __always_inline long syscall_trace_enter(struct pt_regs *regs, unsigned long work, unsigned long *syscall)
> {
>-	long syscall, ret = 0;
>-
> 	/*
> 	 * Handle Syscall User Dispatch.  This must comes first, since
> 	 * the ABI here can be something that doesn't make sense for
>@@ -93,27 +91,25 @@ static __always_inline long syscall_trace_enter(struct pt_regs *regs, unsigned l
> 
> 	/* Handle ptrace */
> 	if (work & (SYSCALL_WORK_SYSCALL_TRACE | SYSCALL_WORK_SYSCALL_EMU)) {
>-		ret = arch_ptrace_report_syscall_entry(regs);
>-		if (ret || (work & SYSCALL_WORK_SYSCALL_EMU))
>+		if (arch_ptrace_report_syscall_entry(regs) || (work & SYSCALL_WORK_SYSCALL_EMU))
> 			return -1L;
> 	}
> 
> 	/* Do seccomp after ptrace, to catch any tracer changes. */
> 	if (work & SYSCALL_WORK_SECCOMP) {
>-		ret = __secure_computing();
>-		if (ret == -1L)
>-			return ret;
>+		if (__secure_computing())
>+			return -1L;
> 	}
> 
> 	/* Either of the above might have changed the syscall number */
>-	syscall = syscall_get_nr(current, regs);
>+	*syscall = syscall_get_nr(current, regs);
> 
> 	if (unlikely(work & SYSCALL_WORK_SYSCALL_TRACEPOINT))
>-		syscall = trace_syscall_enter(regs, syscall);
>+		*syscall = trace_syscall_enter(regs, *syscall);
> 
>-	syscall_enter_audit(regs, syscall);
>+	syscall_enter_audit(regs, *syscall);
> 
>-	return ret ? : syscall;
>+	return 0;
> }
> 
> /**
>@@ -126,12 +122,12 @@ static __always_inline long syscall_trace_enter(struct pt_regs *regs, unsigned l
>  * enabled after invoking enter_from_user_mode(), enabling interrupts and
>  * extra architecture specific work.
>  *
>- * Returns: The original or a modified syscall number
>+ * Returns: The original or a modified syscall number as syscall
>  *
>- * If the returned syscall number is -1 then the syscall should be
>- * skipped. In this case the caller may invoke syscall_set_error() or
>- * syscall_set_return_value() first.  If neither of those are called and -1
>- * is returned, then the syscall will fail with ENOSYS.
>+ * If the returned value is -1 then the syscall should be skipped. In this case
>+ * the caller may invoke syscall_set_error() or syscall_set_return_value()
>+ * first.  If neither of those are called and -1 is returned, then the syscall
>+ * will fail with ENOSYS.
>  *
>  * It handles the following work items:
>  *
>@@ -139,14 +135,14 @@ static __always_inline long syscall_trace_enter(struct pt_regs *regs, unsigned l
>  *     ptrace_report_syscall_entry(), __secure_computing(), trace_sys_enter()
>  *  2) Invocation of audit_syscall_entry()
>  */
>-static __always_inline long syscall_enter_from_user_mode_work(struct pt_regs *regs, long syscall)
>+static __always_inline long syscall_enter_from_user_mode_work(struct pt_regs *regs, long *syscall)
> {
> 	unsigned long work = READ_ONCE(current_thread_info()->syscall_work);
> 
> 	if (work & SYSCALL_WORK_ENTER)
>-		syscall = syscall_trace_enter(regs, work);
>+		return syscall_trace_enter(regs, work, syscall);
> 
>-	return syscall;
>+	return 0;
> }
> 
> /**
>@@ -167,7 +163,7 @@ static __always_inline long syscall_enter_from_user_mode_work(struct pt_regs *re
>  * Returns: The original or a modified syscall number. See
>  * syscall_enter_from_user_mode_work() for further explanation.
>  */
>-static __always_inline long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall)
>+static __always_inline long syscall_enter_from_user_mode(struct pt_regs *regs, long *syscall)
> {
> 	long ret;
> 

Negative numbers most definitely not be assigned as valid system calls, not now, not ever. 

Therein lies some serious madness.

I believe setting the syscall number to -1 to skip is an ABI already in e.g. ptrace, so I doubt we can just get rid of it anyway. 

I would say as follows:

Let's formally define that: 

- valid system call numbers are positive 32-bit numbers, using the appropriate ABI convention for "int".

- bits [30:n] for some value of n are reserved for architecture-specific flags/modes. MIPS uses an offset of 2000 decimal between its syscall ABIs, which would imply n ~ 11, although I personally think that is too restrictive (MIPS could in fact use such a flag to provide an escape into a larger number space if we ever need more than 2000 system calls.)

I would suggest n = 24, at least for now. It is easier to give up additional bits later than to claw them back when already used. 

Thus: 

1. The type for a system call is int.

2. A valid system call number is always going to be positive.

3. Bits [30:24] are available for architecture ABI use. The "architecture independent" part of the system call number is therefore 24 bits wide.

4. The exact ABI is platform-specific, obviously, but as a general guideline (especially for new platforms/ABIs) should follow the rules for a platform "int" if practical. Notably, when passing a value in a register larger than 32 bits, which side of the calling interface is responsible for sign-extending a value passed in a register. If caller side, the kernel should validate, if callee side the kernel should ignore the additional bits and do the extension.

5. A negative system call number is guaranteed to return -ENOSYS (unless intercepted by seccomp, ptrace, or another mechanism under user space control.)

6. If the platform needs to algorithmically modify the system call number due to platform-specific concerns (say, the platform uses a 16-bit special purpose register for the syscall number, or it has multiple kernel entry points with different behavior), it should if at all possible transcode the system call number as necessary to match this convention in APIs that are exposed to general kernel code. 

For example, in the future I could very much see the IA32 code in the x86 kernel using bit 29 internally to indicate an ia32 system call, simplifying the is_compat implementation on x86. It should not mean that passing bit 29 to either the syscall instruction or int $0x80 will be accepted.

Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR

[Sven Schnelle]

Michal Suchánek <msuchanek@suse.de> writes:

> The return value of syscall_enter_from_user_mode is used both for the
> adjusted syscall number and the indicator that a syscall should be
> skipped.
>
> As seccomp can be invoked on any syscall, including invalid ones this
> somewhat undermines seccomp.
>
> While the seccomp variants that terminate the process do not need to
> care about this for the filter that sets the syscall return value this
> disctinction is required.
>
> Pass the syscall number as a pointer to the inline entry functions, and
> use the return value exclusively for the indication that the syscall is
> already handled.
>
> This should avoid the need for the s390 PIF_SYSCALL_RET_SET which is the
> workaround for exactly this deficiency.

I'm not sure whether PIF_SYSCALL_RET_SET can be removed - the syscall
return might still get set by PTRACE_SET_SYSCALL_INFO when the tracee is
stopped. This might be a positive number which can't be distinguished
from a syscall number. But maybe i'm missing something? It's been quite
a while since I touched all that ptrace stuff.

Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR

[Michal Suchánek]

On Thu, Jul 02, 2026 at 10:12:35AM +0200, Sven Schnelle wrote:
> Michal Suchánek <msuchanek@suse.de> writes:
> 
> > The return value of syscall_enter_from_user_mode is used both for the
> > adjusted syscall number and the indicator that a syscall should be
> > skipped.
> >
> > As seccomp can be invoked on any syscall, including invalid ones this
> > somewhat undermines seccomp.
> >
> > While the seccomp variants that terminate the process do not need to
> > care about this for the filter that sets the syscall return value this
> > disctinction is required.
> >
> > Pass the syscall number as a pointer to the inline entry functions, and
> > use the return value exclusively for the indication that the syscall is
> > already handled.
> >
> > This should avoid the need for the s390 PIF_SYSCALL_RET_SET which is the
> > workaround for exactly this deficiency.
> 
> I'm not sure whether PIF_SYSCALL_RET_SET can be removed - the syscall
> return might still get set by PTRACE_SET_SYSCALL_INFO when the tracee is
> stopped. This might be a positive number which can't be distinguished
> from a syscall number. But maybe i'm missing something? It's been quite
> a while since I touched all that ptrace stuff.

When the syscall return value is set (in the registers) the return value
which is also the modified syscall number is set to -1 indicating the
syscall was handled. At least that's how the API is described.

So yes, if the syscall number range is restricted or the syscall number
is returned through a path different from the function return value the
flag should not be needed in the entry path because the case can be
detected through the return value alone.

Thanks

Michal

Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR

[Michal Suchánek]

On Wed, Jul 01, 2026 at 11:29:01AM -0700, H. Peter Anvin wrote:
> On July 1, 2026 10:42:08 AM PDT, "Michal Suchánek" <msuchanek@suse.de> wrote:
> >The return value of syscall_enter_from_user_mode is used both for the
> >adjusted syscall number and the indicator that a syscall should be
> >skipped.
> >
> >As seccomp can be invoked on any syscall, including invalid ones this
> >somewhat undermines seccomp.
> >
> >While the seccomp variants that terminate the process do not need to
> >care about this for the filter that sets the syscall return value this
> >disctinction is required.
> >
> >Pass the syscall number as a pointer to the inline entry functions, and
> >use the return value exclusively for the indication that the syscall is
> >already handled.
> >
> >This should avoid the need for the s390 PIF_SYSCALL_RET_SET which is the
> >workaround for exactly this deficiency.
> >
> >If this is desirable the patch could be split into some series that
> >adjusts the code flow where needed so that the final change is mostly
> >mechanical.
> >
> >There is also another way to handle this problem.
> >
> >With x86 using bit 30 to denote compatibility syscall it sounds like
> >declaring syscall number a 30bit quantity would work.
> >
> >Then bit 31 could be used to denote an invalid syscall that can never be
> >executed, and the -1 returned from syscall_enter_from_user_mode would
> >then be inherently invalid.
> >
> >That is so long as no architectures use syscall numbers outside of this
> >range so far, and the limitation is considered fine.
> > 
> 
> Negative numbers most definitely not be assigned as valid system calls, not now, not ever. 

Negativity of a number is a matter of intepretation. Sometimes the
syscall number is decleared as int, sometimes long, sometimes unsigned
long.

Passing -1 to strtoul generates some bit pattern that can then be
compared to another bit pattern inside a seccomp filter program, for
example.

> Therein lies some serious madness.
> 
> I believe setting the syscall number to -1 to skip is an ABI already in e.g. ptrace, so I doubt we can just get rid of it anyway. 

Yes, and seccomp can set the syscall number to -1 indicating it was
handled already even if the number was -1 to start with. While -1 is not
a valid syscall number it can still be filtered, at least on some
architectures.

> I would say as follows:
> 
> Let's formally define that: 
> 
> - valid system call numbers are positive 32-bit numbers, using the appropriate ABI convention for "int".
> 
> - bits [30:n] for some value of n are reserved for architecture-specific flags/modes. MIPS uses an offset of 2000 decimal between its syscall ABIs, which would imply n ~ 11, although I personally think that is too restrictive (MIPS could in fact use such a flag to provide an escape into a larger number space if we ever need more than 2000 system calls.)
> 
> I would suggest n = 24, at least for now. It is easier to give up additional bits later than to claw them back when already used. 
> 
> Thus: 
> 
> 1. The type for a system call is int.
> 
> 2. A valid system call number is always going to be positive.
> 
> 3. Bits [30:24] are available for architecture ABI use. The "architecture independent" part of the system call number is therefore 24 bits wide.

Will that also work correctly with seccomp?

As I understand it the current situation is that on x86 the BPF code
passed to seccomp must filter the compat syscall bit in the PBF code,
and I do not see how restricting the syscall value to 24bit would happen
without changing the seccomp filter API.

See eg. https://lore.kernel.org/linuxppc-dev/akTExSO3ZT7iRtBa@kunlun.suse.cz/
for sample code.

> 
> 4. The exact ABI is platform-specific, obviously, but as a general guideline (especially for new platforms/ABIs) should follow the rules for a platform "int" if practical. Notably, when passing a value in a register larger than 32 bits, which side of the calling interface is responsible for sign-extending a value passed in a register. If caller side, the kernel should validate, if callee side the kernel should ignore the additional bits and do the extension.

Do we even want to play with sign-extend?

If the syscall number is >= 1<<n after masking off flags recognized by
the platfrom (if any) it's invalid.

> 5. A negative system call number is guaranteed to return -ENOSYS (unless intercepted by seccomp, ptrace, or another mechanism under user space control.)

Interception by seccomp is exactly the case that's wonky.

> 6. If the platform needs to algorithmically modify the system call number due to platform-specific concerns (say, the platform uses a 16-bit special purpose register for the syscall number, or it has multiple kernel entry points with different behavior), it should if at all possible transcode the system call number as necessary to match this convention in APIs that are exposed to general kernel code. 
> 
> For example, in the future I could very much see the IA32 code in the x86 kernel using bit 29 internally to indicate an ia32 system call, simplifying the is_compat implementation on x86. It should not mean that passing bit 29 to either the syscall instruction or int $0x80 will be accepted.

As I understand the code it uses bit 30 for that. Maybe I missed
something?

Thanks

Michal

Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR

[Thomas Gleixner]

On Wed, Jul 01 2026 at 19:42, Michal Suchánek wrote:
> The return value of syscall_enter_from_user_mode is used both for the
> adjusted syscall number and the indicator that a syscall should be
> skipped.
>
> As seccomp can be invoked on any syscall, including invalid ones this
> somewhat undermines seccomp.
>
> While the seccomp variants that terminate the process do not need to
> care about this for the filter that sets the syscall return value this
> disctinction is required.

You completely fail to explain why and what actual problem you are
trying to solve. At least I can't figure it out from the above word
salad.

> Pass the syscall number as a pointer to the inline entry functions, and
> use the return value exclusively for the indication that the syscall is
> already handled.
>
> This should avoid the need for the s390 PIF_SYSCALL_RET_SET which is the
> workaround for exactly this deficiency.
>
> If this is desirable the patch could be split into some series that
> adjusts the code flow where needed so that the final change is mostly
> mechanical.

That's not a matter of desire. That's mandatory.

> -	instrumentation_begin();
> -	if (!invoke_syscall(regs, nr) && nr != -1)
> -	 	result_reg(regs) = __sys_ni_syscall(regs);
> -	instrumentation_end();
> +	/* Skip syscall when -1 is returned */
> +	if (!syscall_enter_from_user_mode(regs, &nr)) {

Seriously?

If we go and separate the syscall number from the return value, then the
return value 0 means success and anything else fail. Which in other
words is a boolean. So instead of tastelessly adding a completely
nonsensical comment about -1 here, syscall_enter_from_user_mode() wants
to have the return value type bool with a proper boolean logic: true =
success, false = abort.

> @@ -168,8 +168,7 @@ __visible noinstr void do_int80_emulation(struct pt_regs *regs)
>  	nr = syscall_32_enter(regs);
>  
>  	local_irq_enable();
> -	nr = syscall_enter_from_user_mode_work(regs, nr);
> -	do_syscall_32_irqs_on(regs, nr);
> +	syscall_enter_from_user_mode_work(regs, &nr);

How exactly is this ever going to invoke a valid syscall?

> +	if (!syscall_enter_from_user_mode_work(regs, &nr)) {
> +		nr &= GENMASK(31, 0);
> +		do_syscall_32_irqs_on(regs, nr);

  do_syscall_32_irqs_on(regs, (int)nr);

would be too simple, right?

Thanks,

        tglx

Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR

[Michal Suchánek]

On Thu, Jul 02, 2026 at 01:24:57PM +0200, Thomas Gleixner wrote:
> On Wed, Jul 01 2026 at 19:42, Michal Suchánek wrote:
> > The return value of syscall_enter_from_user_mode is used both for the
> > adjusted syscall number and the indicator that a syscall should be
> > skipped.
> >
> > As seccomp can be invoked on any syscall, including invalid ones this
> > somewhat undermines seccomp.
> >
> > While the seccomp variants that terminate the process do not need to
> > care about this for the filter that sets the syscall return value this
> > disctinction is required.
> 
> You completely fail to explain why and what actual problem you are
> trying to solve. At least I can't figure it out from the above word
> salad.

syscall_enter_from_user_mode returns the new syscall number after doing
something arbitrarry with it, including running seccomp.

Wehn the syscall is already handled, eg. by seccomp filtering it returns
-1 as the new syscall number. -1 is an invalid syscall number but it can
still be filtered by seccomp. When the syscall number was -1 to start
with it's not possible to determine if the syscall was fileterd from the
return value. s390 returns the filtered state in a flag it sets on the
regs structure, avoiding this problem.

However, the API should be specified in a way that does not require
everyone implementing such flag.

> 
> > Pass the syscall number as a pointer to the inline entry functions, and
> > use the return value exclusively for the indication that the syscall is
> > already handled.
> >
> > This should avoid the need for the s390 PIF_SYSCALL_RET_SET which is the
> > workaround for exactly this deficiency.
> >
> > If this is desirable the patch could be split into some series that
> > adjusts the code flow where needed so that the final change is mostly
> > mechanical.
> 
> That's not a matter of desire. That's mandatory.

So long as it's desirable to implement an API change in this direction,
it's not clear to me so far.

> > -	instrumentation_begin();
> > -	if (!invoke_syscall(regs, nr) && nr != -1)
> > -	 	result_reg(regs) = __sys_ni_syscall(regs);
> > -	instrumentation_end();
> > +	/* Skip syscall when -1 is returned */
> > +	if (!syscall_enter_from_user_mode(regs, &nr)) {
> 
> Seriously?
> 
> If we go and separate the syscall number from the return value, then the
> return value 0 means success and anything else fail. Which in other
> words is a boolean. So instead of tastelessly adding a completely
> nonsensical comment about -1 here, syscall_enter_from_user_mode() wants
> to have the return value type bool with a proper boolean logic: true =
> success, false = abort.

We have that very same API down to __secure_computing() which returns
boolean represented as -1 and 0 values. That does not mean it's not
tasteless.

> 
> > @@ -168,8 +168,7 @@ __visible noinstr void do_int80_emulation(struct pt_regs *regs)
> >  	nr = syscall_32_enter(regs);
> >  
> >  	local_irq_enable();
> > -	nr = syscall_enter_from_user_mode_work(regs, nr);
> > -	do_syscall_32_irqs_on(regs, nr);
> > +	syscall_enter_from_user_mode_work(regs, &nr);
> 
> How exactly is this ever going to invoke a valid syscall?

That's one of the problems with giant all-in-one patch, things like this
easily slip in. However, it is in cluded mostly for illustration, I
don't expect anyone to merge this as-is.

> 
> > +	if (!syscall_enter_from_user_mode_work(regs, &nr)) {
> > +		nr &= GENMASK(31, 0);
> > +		do_syscall_32_irqs_on(regs, nr);
> 
>   do_syscall_32_irqs_on(regs, (int)nr);
> 
> would be too simple, right?

Also way less explicit.

Thanks

Michal

Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR

[Sven Schnelle]

Michal Suchánek <msuchanek@suse.de> writes:

> On Thu, Jul 02, 2026 at 10:12:35AM +0200, Sven Schnelle wrote:
>> Michal Suchánek <msuchanek@suse.de> writes:
>> 
>> > The return value of syscall_enter_from_user_mode is used both for the
>> > adjusted syscall number and the indicator that a syscall should be
>> > skipped.
>> >
>> > As seccomp can be invoked on any syscall, including invalid ones this
>> > somewhat undermines seccomp.
>> >
>> > While the seccomp variants that terminate the process do not need to
>> > care about this for the filter that sets the syscall return value this
>> > disctinction is required.
>> >
>> > Pass the syscall number as a pointer to the inline entry functions, and
>> > use the return value exclusively for the indication that the syscall is
>> > already handled.
>> >
>> > This should avoid the need for the s390 PIF_SYSCALL_RET_SET which is the
>> > workaround for exactly this deficiency.
>> 
>> I'm not sure whether PIF_SYSCALL_RET_SET can be removed - the syscall
>> return might still get set by PTRACE_SET_SYSCALL_INFO when the tracee is
>> stopped. This might be a positive number which can't be distinguished
>> from a syscall number. But maybe i'm missing something? It's been quite
>> a while since I touched all that ptrace stuff.
>
> When the syscall return value is set (in the registers) the return value
> which is also the modified syscall number is set to -1 indicating the
> syscall was handled. At least that's how the API is described.
>
> So yes, if the syscall number range is restricted or the syscall number
> is returned through a path different from the function return value the
> flag should not be needed in the entry path because the case can be
> detected through the return value alone.

I'm still failing to see how this would work without an additional
flag. Assume a program (the tracee) is stopped because of a syscall
entry. The tracer then decides to skip the syscall and changes
regs->gpr2 (which contains either the syscall number or return value)
to contain 42. When the tracer than restarts the syscall, how does
do_syscall() know that gpr2 is now a return value and not a syscall number?

Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR

[Michal Suchánek]

On Thu, Jul 02, 2026 at 02:01:02PM +0200, Sven Schnelle wrote:
> Michal Suchánek <msuchanek@suse.de> writes:
> 
> > On Thu, Jul 02, 2026 at 10:12:35AM +0200, Sven Schnelle wrote:
> >> Michal Suchánek <msuchanek@suse.de> writes:
> >> 
> >> > The return value of syscall_enter_from_user_mode is used both for the
> >> > adjusted syscall number and the indicator that a syscall should be
> >> > skipped.
> >> >
> >> > As seccomp can be invoked on any syscall, including invalid ones this
> >> > somewhat undermines seccomp.
> >> >
> >> > While the seccomp variants that terminate the process do not need to
> >> > care about this for the filter that sets the syscall return value this
> >> > disctinction is required.
> >> >
> >> > Pass the syscall number as a pointer to the inline entry functions, and
> >> > use the return value exclusively for the indication that the syscall is
> >> > already handled.
> >> >
> >> > This should avoid the need for the s390 PIF_SYSCALL_RET_SET which is the
> >> > workaround for exactly this deficiency.
> >> 
> >> I'm not sure whether PIF_SYSCALL_RET_SET can be removed - the syscall
> >> return might still get set by PTRACE_SET_SYSCALL_INFO when the tracee is
> >> stopped. This might be a positive number which can't be distinguished
> >> from a syscall number. But maybe i'm missing something? It's been quite
> >> a while since I touched all that ptrace stuff.
> >
> > When the syscall return value is set (in the registers) the return value
> > which is also the modified syscall number is set to -1 indicating the
> > syscall was handled. At least that's how the API is described.
> >
> > So yes, if the syscall number range is restricted or the syscall number
> > is returned through a path different from the function return value the
> > flag should not be needed in the entry path because the case can be
> > detected through the return value alone.
> 
> I'm still failing to see how this would work without an additional
> flag. Assume a program (the tracee) is stopped because of a syscall
> entry. The tracer then decides to skip the syscall and changes
> regs->gpr2 (which contains either the syscall number or return value)
> to contain 42. When the tracer than restarts the syscall, how does
> do_syscall() know that gpr2 is now a return value and not a syscall number?

Because then the return value from the syscall_enter_from_user_mode
machinery would be -1 indicating the syscall should be skipped. That is
how the return value of syscall_enter_from_user_mode is documented, I
did not verify that it actually works that way for the tracing case on
s390.

So long as it is clarified that -1 is not a syscall number or the
syscall number is retuned elsewhere there is no doubt, the -1 indicates
already handled syscall without the need for an additional flag.

Thanks

Michal