arm64: syscall: Expand the comment about ptrace and syscall(-1) (139dbe5d) · Commits · 戴 / test

If a task executes syscall(-1), we intercept this early and force x0 to be -ENOSYS so that we don't need to distinguish this scenario from one where the scno is -1 because a tracer wants to skip the system call using ptrace. With the return value set, the return path is the same as the skip case. Although there is a one-line comment noting this in el0_svc_common(), it misses out most of the detail. Expand the comment to describe a bit more about what is going on. Cc: Mark Rutland <mark.rutland@arm.com> Cc: Keno Fischer <keno@juliacomputing.com> Cc: Luis Machado <luis.machado@linaro.org> Signed-off-by:

Will Deacon <will@kernel.org>

arch/arm64/kernel/syscall.c

+15 −1

Original line number	Diff line number	Diff line
		@@ -124,7 +124,21 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
		user_exit();

		if (has_syscall_work(flags)) {
		/* set default errno for user-issued syscall(-1) */
		/*
		* The de-facto standard way to skip a system call using ptrace
		* is to set the system call to -1 (NO_SYSCALL) and set x0 to a
		* suitable error code for consumption by userspace. However,
		* this cannot be distinguished from a user-issued syscall(-1)
		* and so we must set x0 to -ENOSYS here in case the tracer doesn't
		* issue the skip and we fall into trace_exit with x0 preserved.
		*
		* This is slightly odd because it also means that if a tracer
		* sets the system call number to -1 but does not initialise x0,
		* then x0 will be preserved for all system calls apart from a
		* user-issued syscall(-1). However, requesting a skip and not
		* setting the return value is unlikely to do anything sensible
		* anyway.
		*/
		if (scno == NO_SYSCALL)
		regs->regs[0] = -ENOSYS;
		scno = syscall_trace_enter(regs);

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