Re: [PATCH v8 2/6] arm64: ptrace: allow tracer to skip a system call

From: AKASHI Takahiro
Date: Thu Nov 20 2014 - 00:13:17 EST

On 11/20/2014 04:06 AM, Will Deacon wrote:
On Wed, Nov 19, 2014 at 08:46:19AM +0000, AKASHI Takahiro wrote:
On 11/18/2014 11:04 PM, Will Deacon wrote:
On Tue, Nov 18, 2014 at 01:10:34AM +0000, AKASHI Takahiro wrote:

+ if (((int)regs->syscallno == -1) && (orig_syscallno == -1)) {
+ /*
+ * user-issued syscall(-1):
+ * RESTRICTION: We always return ENOSYS whatever value is
+ * stored in x0 (a return value) at this point.
+ * Normally, with ptrace off, syscall(-1) returns -ENOSYS.
+ * With ptrace on, however, if a tracer didn't pay any
+ * attention to user-issued syscall(-1) and just let it go
+ * without a hack here, it would return a value in x0 as in
+ * other system call cases. This means that this system call
+ * might succeed and see any bogus return value.
+ * This should be definitely avoided.
+ */
+ regs->regs[0] = -ENOSYS;
+ }

I'm still really uncomfortable with this, and it doesn't seem to match what
arch/arm/ does either.

Yeah, I know but
as I mentioned before, syscall(-1) will be signaled on arm, and so we don't
have to care about a return value :)

What does x86 do?

On x86, syscall(-1) returns -ENOSYS if not traced, and we can change a return
value if traced.

Doesn't it also prevent a tracer from skipping syscall(-1)?

Syscall(-1) will return -ENOSYS whether or not a syscallno is explicitly
replaced with -1 by a tracer, and, in this sense, it is *skipped*.

Ok, but now userspace sees -ENOSYS for a skipped system call in that case,
whereas it would usually see whatever the trace put in x0, right?

If you don't really like this behavior, how about this patch instead of my [2/6] patch?

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 726b910..1ef57d0 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -668,8 +668,15 @@ ENDPROC(el0_svc)
* switches, and waiting for our parent to respond.
+ cmp w8, #-1 // default errno for invalid
+ 1f // system call
+ mov x0, #-ENOSYS
+ str x0, [sp, #S_X0]
mov x0, sp
bl syscall_trace_enter
+ cmp w0, #-1 // skip the syscall?
+ b.eq __sys_trace_return_skipped
adr lr, __sys_trace_return // return address
uxtw scno, w0 // syscall number (possibly new)
mov x1, sp // pointer to regs
@@ -684,6 +691,7 @@ __sys_trace:

str x0, [sp] // save returned x0
mov x0, sp
bl syscall_trace_exit
b ret_to_user

With this change, I believe, syscall(-1) returns -ENOSYS by default whether traced
or not, and still you can change a return value when tracing.
(But a drawback here is that a tracer will see -ENOSYS in x0 even at syscall entry
for syscall(-1).)

-Takahiro AKASHI


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