Messages in this thread | | | From | Keno Fischer <> | Date | Fri, 22 May 2020 21:01:01 -0400 | Subject | ptrace: seccomp: Return value when the call was already invalid |
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I'm seeing the following while porting a ptracer from x86_64 to arm64 (cc'ing arm64 folks, but in this case x86_64 is the odd one out, I think other archs would be consistent with arm64).
Consider userspace code like the following: ``` int ret = syscall(-10, 0); assert(ret == -ENOSYS); ```
(Never mind the fact that this is something userspace shouldn't do, I saw this in our test suite that tests corner cases where the ptracer shouldn't affect behavior).
Now, if we have a seccomp filter that simply does SECCOMP_RET_TRACE, and a ptracer that simply does PTRACE_CONT, then the assert will fire/fail on arm64, but not on x86_64.
The reason this happens is that the return value gets set early on x86_64, but this is not possible on arm64, because doing so would clobber the first argument register that it shares. As a result, no return value is set and `ret` retains the value that the first syscall argument used to have.
I can work around this of course, but I guess my question is whether this is expected/ok, or you would expect an active ptracer that does not touch the registers not to affect behavior.
Interestingly, arm64 does do something different if the syscall is -1 rather than -10, where early in the ptrace stop it does. ``` /* set default errno for user-issued syscall(-1) */ if (scno == NO_SYSCALL) regs->regs[0] = -ENOSYS; ```
I'm not sure that's great either since the ptracer may want to inspect x0 and arm64 does not make orig_x0 available via ptrace. To me this indicates that maybe this was intended to apply to any syscall skipped here, not just -1 (the different comes from the fact that seccomp considers any negative syscall a skip/fail, but on syscall-entry stops arm64 only considers a literal -1 a skip).
On the other hand if this is deemed expected, I'll go ahead and submit a man-page patch to at least document this architecture difference.
Keno
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