diff options
author | Will Deacon <will@kernel.org> | 2020-07-03 09:41:24 +0100 |
---|---|---|
committer | Will Deacon <will@kernel.org> | 2020-07-16 11:41:58 +0100 |
commit | 139dbe5d8ed383cbd1ada56c78dbbbd35bf6a9d3 (patch) | |
tree | 7ab72588884da81954b1f9f882795c80dd556cd6 /arch/arm64/kernel/syscall.c | |
parent | 59ee987ea47caff8c1e7ba4b89932c6900a35d0c (diff) |
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>
Diffstat (limited to 'arch/arm64/kernel/syscall.c')
-rw-r--r-- | arch/arm64/kernel/syscall.c | 16 |
1 files changed, 15 insertions, 1 deletions
diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c index 98a26d4e7b0c..5f0c04863d2c 100644 --- 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); |