linux-kernel - Re: [PATCH v3 0/5] nolibc signal handling support

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Message-ID: <CAOG64qMcvd==jfHc22q2=n79siixqbKOxb7N8i6SXjqCJtnZpw@mail.gmail.com>
Date:   Mon, 16 Jan 2023 00:06:44 +0700
From:   Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>
Cc:     Willy Tarreau <w@....eu>, Shuah Khan <shuah@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Gilang Fachrezy <gilang4321@...il.com>,
        "GNU/Weeb Mailing List" <gwml@...r.gnuweeb.org>,
        VNLX Kernel Department <kernel@...x.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Kselftest Mailing List 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v3 0/5] nolibc signal handling support

On Sun, Jan 15, 2023 at 11:01 PM Ammar Faizi wrote:
> That is not the 'sigset_t' that the kernel wants. The kernel wants the
> 'sigset_t' that is in <asm-generic/signal.h>:
>
>     #define _NSIG       64
>     #define _NSIG_BPW   __BITS_PER_LONG      // this 64 on x86-64, but 32 on i386.
>     #define _NSIG_WORDS (_NSIG / _NSIG_BPW)
>
>     typedef struct {
>         unsigned long sig[_NSIG_WORDS];
>     } sigset_t;
>
> The above struct is always 8 bytes in size. In other words:
>
>     _NSIG_WORDS == 2 on i386
>     _NSIG_WORDS == 1 on x86-64
>     sizeof(unsigned long) == 4 on i386
>     sizeof(unsigned long) == 8 on x86-64
>
> Therefore, sizeof(unsigned long [_NSIG_WORDS]) is always 8 on both
> architectures. That's the correct size.
>
> I tried to #include <asm-generic/signal.h> but it conflicts with the
> other 'sigset_t' definition. So I can't do that.

Read the glibc sigaction implementation, it has different struct
sigaction definitions for user and kernel too.

https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/libc_sigaction.c;h=3cbf241a5fa28296c910fa40a41b09d2b6113b05;hb=7e31d166510ac4adbf53d5e8144c709a37dd8c7a

Since nolibc compiles everything in a single translation unit, you
can't have a different sigset_t definition. You need to copy the
definition to nolibc header and change the type name to something else
for internal use only.

> Why are there two different definitions of 'sigset_t'? I don't know.
>
> I probably should read the story behind this syscall to get it
> implemented right. Let me ponder this again on Monday. But at least I
> tell what I have found so people can give some comments on it...

`man 2 rt_sigaction` tells the story. Here is the bedtime story, have
a nice dream :-)

The original Linux system call was named sigaction(). However, with
the addition of real-time signals in Linux 2.2, the fixed-size, 32-bit
sigset_t type supported by that system call was no longer fit for
purpose.
Consequently, a new system call, rt_sigaction(), was added to support
an enlarged sigset_t type. The new system call takes a fourth
argument, size_t sigsetsize, which specifies the size in bytes of the
signal sets
in act.sa_mask and oldact.sa_mask. This argument is currently required
to have the value sizeof(sigset_t) (or the error EINVAL results). The
glibc sigaction() wrapper function hides these details from us,
transpar‐
ently calling rt_sigaction() when the kernel provides it.