linux-kernel - Re: [Bug Report] Wrong value of __NR_userfaultfd in asm-generic/unistd.h

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Message-ID: <4a775fd6-cb07-46eb-aa15-026e61317c16@redhat.com>
Date: Mon, 21 Oct 2024 11:33:57 +0200
From: David Hildenbrand <david@...hat.com>
To: Muhammad Usama Anjum <Usama.Anjum@...labora.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>,
 Peter Xu <peterx@...hat.com>, "Dr. David Alan Gilbert"
 <dgilbert@...hat.com>, Andrea Arcangeli <aarcange@...hat.com>,
 Kim Phillips <kim.phillips@....com>
Cc: kernel@...labora.com, linux-mm@...ck.org,
 linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
 John Hubbard <jhubbard@...dia.com>, Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [Bug Report] Wrong value of __NR_userfaultfd in
 asm-generic/unistd.h

Am 21.10.24 um 08:48 schrieb Muhammad Usama Anjum:
> Hi,
> 
> The asm-generic/unistd.h file has wrong __NR_userfaultfd syscall number which
> doesn't even depend on the architecture. This has caused failure of a selftest
> which was fixed recently [1].
> 
> grep -rnIF "#define __NR_userfaultfd"
> tools/include/uapi/asm-generic/unistd.h:681:#define __NR_userfaultfd 282
> arch/x86/include/generated/uapi/asm/unistd_32.h:374:#define __NR_userfaultfd 374
> arch/x86/include/generated/uapi/asm/unistd_64.h:327:#define __NR_userfaultfd 323
> arch/x86/include/generated/uapi/asm/unistd_x32.h:282:#define __NR_userfaultfd (__X32_SYSCALL_BIT + 323)
> arch/arm/include/generated/uapi/asm/unistd-eabi.h:347:#define __NR_userfaultfd (__NR_SYSCALL_BASE + 388)
> arch/arm/include/generated/uapi/asm/unistd-oabi.h:359:#define __NR_userfaultfd (__NR_SYSCALL_BASE + 388)
> include/uapi/asm-generic/unistd.h:681:#define __NR_userfaultfd 282
> 
> The number is dependent on the architecture. The above data shows that it
> is different for different arch:
> x86	374
> x86_64	323
> ARM     347/358
> 
> It seems include/uapi/asm-generic/unistd has wrong 282 value in it. Maybe I'm
> missing some context.. Please have a look at it.
> 
> The __NR_userfaultfd was added to include/uapi/asm-generic/unistd.h in
> 09f7298100ea ("Subject: [PATCH] userfaultfd: register uapi generic syscall (aarch64)").

This is not specific to __NR_userfaultfd, just take a look at some of the other 
syscalls (e.g., __NR_membarrier).

Now, some of the files you list above are "generated". Doing it on a clean tree:

$ grep -rnIF "#define __NR_userfaultfd"
arch/arm64/include/asm/unistd32.h:789:#define __NR_userfaultfd 388
tools/include/uapi/asm-generic/unistd.h:681:#define __NR_userfaultfd 282
include/uapi/asm-generic/unistd.h:681:#define __NR_userfaultfd 282

But now comes the tricky part: an architecture defines whether it wants to

(a) Use the asm-generic unistd.h
(b) Use a custom one

E.g.,

$ cat include/uapi/linux/unistd.h
/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
#ifndef _LINUX_UNISTD_H_
#define _LINUX_UNISTD_H_

/*
  * Include machine specific syscall numbers
  */
#include <asm/unistd.h>

#endif /* _LINUX_UNISTD_H_ */

For example on riscv arch/riscv/include/asm/unistd.h  will include 
arch/riscv/include/uapi/asm/unistd.h which will include "asm-generic/unistd.h".

If you follow the flow on x86, you'll find that it will not include that 
asm-generic one as default.

So the asm-generic variant only applies if an arch wants to do it in the generic 
way.

$ find tools -name unistd.h
tools/arch/x86/include/uapi/asm/unistd.h
tools/arch/arc/include/uapi/asm/unistd.h
tools/arch/riscv/include/uapi/asm/unistd.h
tools/arch/hexagon/include/uapi/asm/unistd.h
tools/arch/arm64/include/uapi/asm/unistd.h
tools/arch/loongarch/include/uapi/asm/unistd.h
tools/include/uapi/asm-generic/unistd.h
tools/include/nolibc/unistd.h

Consequently, the asm-generic one should never be used directly.

-- 
Cheers,

David / dhildenb