linux-kernel - Re: [PATCH] nolibc/stdlib: Improve `getauxval(3)` implementation

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Date: Wed, 17 Jan 2024 02:23:53 +0700
From: Ammar Faizi <ammarfaizi2@...weeb.org>
To: Willy Tarreau <w@....eu>
Cc: Charles Mirabile <cmirabil@...hat.com>, linux-kernel@...r.kernel.org,
	Thomas Weißschuh <linux@...ssschuh.net>
Subject: Re: [PATCH] nolibc/stdlib: Improve `getauxval(3)` implementation

On Wed, Jan 17, 2024 at 02:11:12AM +0700, Ammar Faizi wrote:
> On Tue, Jan 16, 2024 at 07:59:39PM +0100, Willy Tarreau wrote:
> > On Tue, Jan 16, 2024 at 07:58:09PM +0100, Willy Tarreau wrote:
> > > On Wed, Jan 17, 2024 at 01:52:06AM +0700, Ammar Faizi wrote:
> > > > What do you think about other architectures? Will it potentially be
> > > > misinterpreted?
> > > 
> > > Indeed, it would fail on a 64-bit big endian architecture. Let's
> > > just declare the local variable the same way as it is in the spec,
> > > it will be much cleaner and more reliable.
> > 
> > With that said, if previous code used to work on such architectures,
> > maybe the definition above is only for x86_64 and differs on other
> > archs. Maybe it's really defined as two longs ?
> 
> I just took a look at the kernel source code:
> https://github.com/torvalds/linux/blob/v6.7/fs/binfmt_elf.c#L226-L261
> 
> The auxv is stored in `elf_info` variable, the type is `elf_addr_t`. Not
> sure what kind of typedef is that. I'll check.
> 
> Each auxv entry is added using this macro:
> 
>  #define NEW_AUX_ENT(id, val) \
>          do { \
>                  *elf_info++ = id; \
>                  *elf_info++ = val; \
>          } while (0)
> 
> where `id` is the type. That clearly implies `type` and `val` have the
> same size on the Linux kernel.

So here is the result:

1. 'elf_addr_t' defintion ( https://github.com/torvalds/linux/blob/v6.7/include/linux/elf.h#L38-L62 ):

  (simplified)
  #if ELF_CLASS == ELFCLASS32
  #define elf_addr_t	Elf32_Off
  #else
  #define elf_addr_t	Elf64_Off
  #endif

2. 'Elf32_Off' and 'Elf64_Off' typedefs ( https://github.com/torvalds/linux/blob/v6.7/include/uapi/linux/elf.h#L8-L23 )

  typedef __u32  Elf32_Off;
  typedef __u64  Elf64_Off;

Assuming 'ELFCLASS32' is for 32-bit architectures, then it's two __u64
on 64-bit arch, and two __u32 on 32-bit arch. That is identical to
'unsigned long' for both cases (on Linux). So it's fine to have
'unsigned long' for both 'type' and 'value'.

-- 
Ammar Faizi