linux-kernel - Re: pt_regs->ax == -ENOSYS

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Date:   Tue, 27 Apr 2021 19:07:51 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        oleg@...hat.com, Will Drewry <wad@...omium.org>
Subject: Re: pt_regs->ax == -ENOSYS

Earlier in the thread the suggestion was to have (int)pt_regs->orig_ax < 0 indicate a nonsyscall.

On April 27, 2021 7:05:56 PM PDT, Kees Cook <keescook@...omium.org> wrote:
>On Tue, Apr 27, 2021 at 04:51:06PM -0700, Andy Lutomirski wrote:
>> Fortunately there is not, and never will be, a syscall -1.  But I
>> agree that calling max syscall + 1 should behave identically to
>calling
>> a nonexistent syscall in the middle of the table.
>
>If that happens, we have to separate the meaning of -1L from ptrace,
>seccomp, etc. (i.e. we can't just add an "else { result = -ENOSYS; }"
>to
>the syscall table dispatching code, since that'll overwrite any written
>return value when the syscall is meant to be skipped with a specific
>return value set by ptrace/seccomp.
>
>syscall_trace_enter() will currently return either -1 or the
>syscall. Which means someone making a "syscall -1" will get the skip
>semantics currently (though the preloaded -ENOSYS results in the
>"expected" outcome).
>
>arm64 recently had to untangle this too:
>
>15956689a0e6 arm64: compat: Ensure upper 32 bits of x0 are zero on
>syscall return
>59ee987ea47c arm64: ptrace: Add a comment describing our syscall
>entry/exit trap ABI
>139dbe5d8ed3 arm64: syscall: Expand the comment about ptrace and
>syscall(-1)
>d83ee6e3e75d arm64: ptrace: Use NO_SYSCALL instead of -1 in
>syscall_trace_enter()

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