<p></p>
<blockquote>
<p dir="auto">The problematic piece of kernel code is</p>
<pre><code>        if ((nr == -1UL) && (onr != -1UL))
                goto out; /* Don't set -ENOSYS for SECCOMP */
</code></pre>
<p dir="auto">Here the <code>Don't set -ENOSYS for SECCOMP</code> is the culprit: when the syscall is cancelled by ptrace/seccomp, the return value must be set properly to -ENOSYS.</p>
</blockquote>
<p dir="auto">This piece of code is correct for SECCOMP because the latter calls <code>syscall_set_return_value</code> itself.</p>
<p dir="auto">Unfortunately, this approach is not applicable for ptrace:<br>
ptracer cannot set syscall return value on entering syscall<br>
because the register used for the return value is often used for other purposes.<br>
For example, on x86 it's also used for the syscall nr, and on loongarch it's also used for the first syscall argument.</p>
<p dir="auto">I'm afraid <code>syscall_enter_from_user_mode</code> needs to be changed before it could be used on loongarch.<br>
Besides loongarch, the only architecture where <code>syscall_enter_from_user_mode</code> is used<br>
is x86 where <code>rax</code> is initialized to <code>-ENOSYS</code> before <code>syscall_enter_from_user_mode</code> is invoked.</p>

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