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Date: Tue, 16 Jan 2024 20:46:55 +0100
From: Willy Tarreau <w@1wt.eu>
To: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Cc: Charles Mirabile <cmirabil@redhat.com>, linux-kernel@vger.kernel.org,
        Thomas =?iso-8859-1?Q?Wei=DFschuh?= <linux@weissschuh.net>
Subject: Re: [PATCH] nolibc/stdlib: Improve `getauxval(3)` implementation
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On Wed, Jan 17, 2024 at 02:23:53AM +0700, Ammar Faizi wrote:
> 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'.

Yeah I agree, thanks for checking.

Willy