[patch 18/18] entry, treewide: Make syscall_enter_from_user_mode[_work]() indicate syscall execution
Shrikanth Hegde
sshegde at linux.ibm.com
Tue Jul 7 22:21:56 PDT 2026
Hi Thomas.
On 7/8/26 12:37 AM, Thomas Gleixner wrote:
> From: Michal Suchánek <msuchanek at suse.de>
>
> The return values of syscall_enter_from_user_mode[_work]() are
> non-intuitive. Both functions return the syscall number which should be
> invoked by the architecture specific syscall entry code. The returned
> number can be:
>
> - the unmodified syscall number which was handed in by the caller
>
> - a modified syscall number (ptrace, seccomp, trace/probe/bpf)
>
> That has an additional twist. If the return value is -1L then the caller is
> not allowed to modify the return value as that indicates that the modifying
> entity requests to abort the syscall and set the return value already. That
> can obviously not be differentiated from a syscall which handed in -1 as
> syscall number.
>
> The established way to deal with that is:
>
> set_return_value(regs, -ENOSYS);
> nr = syscall_enter_from_user_mode(regs, nr);
> if ((unsigned)nr < SYSCALLNR_MAX)
> handle_syscall(regs, nr);
> else if (nr != -1)
> set_return_value(regs, -ENOSYS);
>
> The latter is obviously redundant, but that's just a leftover of the
> historical evolution of this code. S390 has some special requirements here,
> which can be avoided when the return value is not ambiguous.
>
> Now that the functions which modify the syscall number and want to abort
> are converted to indicate that with a boolean return value, it's obvious to
> hand this through to the callers.
>
> Rework syscall_enter_from_user_mode[_work]) so they take a pointer to the
> syscall number and return a boolean, which indicates whether the syscall
> should be handled or not.
>
> That's not only more intuitive, it also results in slightly denser
> executable code on x86 at least, but perf results are neutral and within
> the noise.
>
> [ tglx: Adopted it to the changes in the generic entry code, fixed up the
> 32-bit fallout and rewrote change log ]
>
> Signed-off-by: Michal Suchánek <msuchanek at suse.de>
> Signed-off-by: Thomas Gleixner <tglx at kernel.org>
> Cc: Jonathan Corbet <corbet at lwn.net>
> Cc: Arnd Bergmann <arnd at arndb.de>
> Cc: Mark Rutland <mark.rutland at arm.com>
> Cc: Huacai Chen <chenhuacai at kernel.org>
> Cc: Michael Ellerman <mpe at ellerman.id.au>
> Cc: Shrikanth Hegde <sshegde at linux.ibm.com>
> Cc: Paul Walmsley <pjw at kernel.org>
> Cc: Palmer Dabbelt <palmer at dabbelt.com>
> Cc: Sven Schnelle <svens at linux.ibm.com>
> Cc: linux-doc at vger.kernel.org
> Cc: loongarch at lists.linux.dev
> Cc: linuxppc-dev at lists.ozlabs.org
> Cc: linux-riscv at lists.infradead.org
> Cc: linux-s390 at vger.kernel.org
> ---
> Documentation/core-api/entry.rst | 18 +++++++++++-------
> arch/loongarch/kernel/syscall.c | 14 +++++++-------
> arch/powerpc/kernel/syscall.c | 3 ++-
> arch/riscv/kernel/traps.c | 11 +++++------
> arch/s390/kernel/syscall.c | 7 +++++--
> arch/x86/entry/syscall_32.c | 25 ++++++++++++-------------
> arch/x86/entry/syscall_64.c | 12 ++++++------
> include/linux/entry-common.h | 12 +++++-------
> 8 files changed, 53 insertions(+), 49 deletions(-)
>
> --- a/Documentation/core-api/entry.rst
> +++ b/Documentation/core-api/entry.rst
> @@ -68,16 +68,20 @@ low-level C code must not be instrumente
> noinstr void syscall(struct pt_regs *regs, int nr)
> {
> arch_syscall_enter(regs);
> - nr = syscall_enter_from_user_mode_randomize_stack(regs, nr);
> -
> - instrumentation_begin();
> - if (!invoke_syscall(regs, nr) && nr != -1)
> - result_reg(regs) = __sys_ni_syscall(regs);
> - instrumentation_end();
> -
> + result_reg(regs) = -ENOSYS;
> + if (syscall_enter_from_user_mode_randomize_stack(regs, &nr)) {
> + instrumentation_begin();
> + if (!invoke_syscall(regs, nr))
> + result_reg(regs) = __sys_ni_syscall(regs);
> + instrumentation_end();
> + }
> syscall_exit_to_user_mode(regs);
> }
>
> +It is required that either the low level assembly code or the syscall
> +function sets the result register to -ENOSYS before invoking the generic
> +code.
> +
> syscall_enter_from_user_mode_randomize_stack() first invokes
> enter_from_user_mode_randomize_stack() which establishes state in the
> following order:
> --- a/arch/loongarch/kernel/syscall.c
> +++ b/arch/loongarch/kernel/syscall.c
> @@ -57,8 +57,8 @@ typedef long (*sys_call_fn)(unsigned lon
>
> void noinstr __no_stack_protector do_syscall(struct pt_regs *regs)
> {
> - unsigned long nr;
> sys_call_fn syscall_fn;
> + unsigned long nr;
>
> nr = regs->regs[11];
> /* Set for syscall restarting */
> @@ -69,12 +69,12 @@ void noinstr __no_stack_protector do_sys
> regs->orig_a0 = regs->regs[4];
> regs->regs[4] = -ENOSYS;
>
> - nr = syscall_enter_from_user_mode_randomize_stack(regs, nr);
> -
> - if (nr < NR_syscalls) {
> - 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]);
> + if (likely(syscall_enter_from_user_mode_randomize_stack(regs, &nr))) {
> + if (nr < NR_syscalls) {
> + 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]);
> + }
> }
>
> syscall_exit_to_user_mode(regs);
> --- a/arch/powerpc/kernel/syscall.c
> +++ b/arch/powerpc/kernel/syscall.c
> @@ -18,7 +18,8 @@ notrace long system_call_exception(struc
> long ret;
> syscall_fn f;
>
> - r0 = syscall_enter_from_user_mode_randomize_stack(regs, r0);
> + if (unlikely(!syscall_enter_from_user_mode_randomize_stack(regs, &r0))
> + return syscall_get_error(current, regs);
>
There is one missing )
diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
index d85894bdb6a2..1440dcabe052 100644
--- a/arch/powerpc/kernel/syscall.c
+++ b/arch/powerpc/kernel/syscall.c
@@ -18,7 +18,7 @@ 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))
+ if (unlikely(!syscall_enter_from_user_mode_randomize_stack(regs, &r0)))
return syscall_get_error(current, regs);
if (unlikely(r0 >= NR_syscalls)) {
> if (unlikely(r0 >= NR_syscalls)) {
> if (unlikely(trap_is_unsupported_scv(regs))) {
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -332,13 +332,12 @@ void do_trap_ecall_u(struct pt_regs *reg
>
> riscv_v_vstate_discard(regs);
>
> - syscall = syscall_enter_from_user_mode_randomize_stack(regs, syscall);
> -
> - if (syscall >= 0 && syscall < NR_syscalls) {
> - syscall = array_index_nospec(syscall, NR_syscalls);
> - syscall_handler(regs, syscall);
> + if (syscall_enter_from_user_mode_randomize_stack(regs, &syscall)) {
> + if (syscall >= 0 && syscall < NR_syscalls) {
> + syscall = array_index_nospec(syscall, NR_syscalls);
> + syscall_handler(regs, syscall);
> + }
> }
> -
> syscall_exit_to_user_mode(regs);
> } else {
> irqentry_state_t state = irqentry_nmi_enter(regs);
> --- a/arch/s390/kernel/syscall.c
> +++ b/arch/s390/kernel/syscall.c
> @@ -96,6 +96,7 @@ SYSCALL_DEFINE0(ni_syscall)
> void noinstr __do_syscall(struct pt_regs *regs, int per_trap)
> {
> unsigned long nr;
> + bool permit;
>
> enter_from_user_mode_randomize_stack(regs);
>
> @@ -121,7 +122,9 @@ void noinstr __do_syscall(struct pt_regs
> regs->psw.addr = current->restart_block.arch_data;
> current->restart_block.arch_data = 1;
> }
> - nr = syscall_enter_from_user_mode_work(regs, nr);
> +
> + permit = 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 +132,7 @@ void noinstr __do_syscall(struct pt_regs
> * 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) || !permit))
> goto out;
> regs->gprs[2] = -ENOSYS;
> if (likely(nr < NR_syscalls)) {
> --- a/arch/x86/entry/syscall_32.c
> +++ b/arch/x86/entry/syscall_32.c
> @@ -161,8 +161,9 @@ static __always_inline bool int80_is_ext
> 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 (likely(syscall_enter_from_user_mode_work(regs, &nr)))
> + do_syscall_32_irqs_on(regs, nr);
>
> instrumentation_end();
> syscall_exit_to_user_mode(regs);
> @@ -223,8 +224,8 @@ 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 (likely(syscall_enter_from_user_mode_work(regs, &nr)))
> + do_syscall_32_irqs_on(regs, nr);
>
> instrumentation_end();
> syscall_exit_to_user_mode(regs);
> @@ -243,13 +244,13 @@ DEFINE_FREDENTRY_RAW(int80_emulation)
> * orig_ax, the int return value truncates it. This matches
> * the semantics of syscall_get_nr().
> */
> - nr = syscall_enter_from_user_mode_randomize_stack(regs, nr);
> -
> - instrumentation_begin();
> + if (likely(syscall_enter_from_user_mode_randomize_stack(regs, &nr))) {
> + instrumentation_begin();
>
> - do_syscall_32_irqs_on(regs, nr);
> + do_syscall_32_irqs_on(regs, nr);
>
> - instrumentation_end();
> + instrumentation_end();
> + }
> syscall_exit_to_user_mode(regs);
> }
> #endif /* !CONFIG_IA32_EMULATION */
> @@ -286,10 +287,8 @@ static noinstr bool __do_fast_syscall_32
> 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 (likely(syscall_enter_from_user_mode_work(regs, &nr)))
> + do_syscall_32_irqs_on(regs, nr);
>
> instrumentation_end();
> syscall_exit_to_user_mode(regs);
> --- a/arch/x86/entry/syscall_64.c
> +++ b/arch/x86/entry/syscall_64.c
> @@ -78,14 +78,14 @@ static __always_inline void do_syscall_x
> /* Returns true to return using SYSRET, or false to use IRET */
> __visible noinstr bool do_syscall_64(struct pt_regs *regs, long nr)
> {
> - nr = syscall_enter_from_user_mode_randomize_stack(regs, nr);
> + if (likely(syscall_enter_from_user_mode_randomize_stack(regs, &nr))) {
> + instrumentation_begin();
>
> - instrumentation_begin();
> + if (!do_syscall_x64(regs, nr))
> + do_syscall_x32(regs, nr);
>
> - if (!do_syscall_x64(regs, nr))
> - do_syscall_x32(regs, nr);
> -
> - instrumentation_end();
> + instrumentation_end();
> + }
> syscall_exit_to_user_mode(regs);
>
> /*
> --- a/include/linux/entry-common.h
> +++ b/include/linux/entry-common.h
> @@ -71,7 +71,7 @@ static inline void syscall_enter_audit(s
> }
> }
>
> -static __always_inline bool syscall_trace_enter(struct pt_regs *regs, unsigned long work,
> +static __always_inline long syscall_trace_enter(struct pt_regs *regs, unsigned long work,
> long *syscall)
> {
> /*
> @@ -141,16 +141,14 @@ static __always_inline bool syscall_trac
> * ptrace_report_syscall_permit_entry(), __seccomp_permit_syscall(), 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 bool 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) {
> - if (!syscall_trace_enter(regs, work, &syscall))
> - return -1L;
> - }
> + if (unlikely(work & SYSCALL_WORK_ENTER))
> + return syscall_trace_enter(regs, work, syscall);
>
> - return syscall;
> + return true;
> }
>
> /**
>
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