linux-kernel - Re: [PATCH v4 0/7] ptrace: introduce PTRACE_SET_SYSCALL

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Message-ID: <20250203103542.GA16165@strace.io>
Date: Mon, 3 Feb 2025 12:35:43 +0200
From: "Dmitry V. Levin" <ldv@...ace.io>
To: Alexander Gordeev <agordeev@...ux.ibm.com>
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Subject: Re: [PATCH v4 0/7] ptrace: introduce PTRACE_SET_SYSCALL_INFO API

On Mon, Feb 03, 2025 at 10:29:37AM +0100, Alexander Gordeev wrote:
> On Mon, Feb 03, 2025 at 08:58:49AM +0200, Dmitry V. Levin wrote:
> 
> Hi Dmitry,
> 
> > PTRACE_SET_SYSCALL_INFO is a generic ptrace API that complements
> > PTRACE_GET_SYSCALL_INFO by letting the ptracer modify details of
> > system calls the tracee is blocked in.
> ...
> 
> FWIW, I am getting these on s390:
> 
> # ./tools/testing/selftests/ptrace/set_syscall_info 
> TAP version 13
> 1..1
> # Starting 1 tests from 1 test cases.
> #  RUN           global.set_syscall_info ...
> # set_syscall_info.c:87:set_syscall_info:Expected exp_entry->nr (-1) == info->entry.nr (65535)
> # set_syscall_info.c:88:set_syscall_info:wait #3: PTRACE_GET_SYSCALL_INFO #2: syscall nr mismatch
> # set_syscall_info: Test terminated by assertion
> #          FAIL  global.set_syscall_info
> not ok 1 global.set_syscall_info
> # FAILED: 0 / 1 tests passed.
> # Totals: pass:0 fail:1 xfail:0 xpass:0 skip:0 error:0
> 
> I remember one of the earlier versions (v1 or v2) was working for me.
> 
> Thanks!

In v3, this test was extended to check whether PTRACE_GET_SYSCALL_INFO
called immediately after PTRACE_SET_SYSCALL_INFO returns the same syscall
number, and on s390 it apparently doesn't, thanks to its implementation
of syscall_get_nr() that returns 0xffff in this case.

To workaround this, we could either change syscall_get_nr() to return -1
in this case, or add an #ifdef __s390x__ exception to the test.

What would you prefer?


-- 
ldv