arm64: syscall: Expand the comment about ptrace and syscall(-1)

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>

diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index 98a26d4e7b0c875af9e0c257ccefe38654f11755..5f0c04863d2c19eaecf2455b9db38eda3ef53eb5 100644 (file)
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -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);