linux-kernel - Re: [RFC PATCH v1 0/8] nolibc signal handling support

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Date:   Tue, 3 Jan 2023 10:51:35 +0700
From:   Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>
Cc:     Willy Tarreau <w@....eu>, "Paul E. McKenney" <paulmck@...nel.org>,
        Shuah Khan <shuah@...nel.org>,
        Gilang Fachrezy <gilang4321@...il.com>,
        VNLX Kernel Department <kernel@...x.org>,
        Kanna Scarlet <knscarlet@...weeb.org>,
        Muhammad Rizki <kiizuha@...weeb.org>,
        "GNU/Weeb Mailing List" <gwml@...r.gnuweeb.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Kselftest Mailing List 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [RFC PATCH v1 0/8] nolibc signal handling support

On Thu, Dec 29, 2022 at 6:42 PM Ammar Faizi wrote:
> On 12/28/22 8:35 PM, Willy Tarreau wrote:
> > It gives me the correct code for x86_64 and i586. I don't know if other
> > architectures will want to add a prologue. I tried with "naked" but it's
> > ignored by the compiler since the function is not purely asm. Not very
> > important but given that we already have everything to perform our calls
> > it would make sense to stay on this. By the way, for the sake of
> > consistency with other syscalls, I do think the function (or label if
> > we can't do otherwise) should be called "sys_rt_sigreturn" as it just
> > performs a syscall.
>
> Will call that 'sys_rt_sigreturn' in the next series.

>From glibc source code says:
GDB needs some intimate knowledge about it to recognize them as signal
trampolines, and make backtraces through signal handlers work right.
Important are both the names (__restore_rt) and the exact instruction
sequence.

link: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/x86_64/sigaction.c;h=4e6d9cc32e1e18746726fa430d092de9a19ba6c6;hb=b4a5d26d8835d972995f0a0a2f805a8845bafa0b#l34

glibc does this:

   "    .type __" #name ",@function\n" \
   "__" #name ":\n"                    \
   "    movq $" #syscall ", %rax\n"    \
   "    syscall\n"                     \

where

   #name = "restore_rt"
   #syscall = __NR_rt_sigreturn

I think it should be called "__restore_rt" instead of "sys_rt_sigreturn"?
glibc also has unwind information, but we probably don't need to care
with that much