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

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Date: Wed, 17 Jan 2024 01:52:06 +0700
From: Ammar Faizi <ammarfaizi2@...weeb.org>
To: Charles Mirabile <cmirabil@...hat.com>
Cc: linux-kernel@...r.kernel.org, Willy Tarreau <w@....eu>,
	Thomas Weißschuh <linux@...ssschuh.net>
Subject: Re: [PATCH] nolibc/stdlib: Improve `getauxval(3)` implementation

On Tue, Jan 16, 2024 at 01:11:47PM -0500, Charles Mirabile wrote:
> At least on x86-64, the ABI only specifies that one more long will be
> present with value 0 (type AT_NULL) after the pairs of auxv entries.
> Whether or not it has a corresponding value is unspecified. This value is
> present on linux, but there is no reason to check it as simply seeing an
> auxv entry whose type value is AT_NULL should be enough.

Yeah, I agree with that. I just read the ABI and confirmed that the
'a_un' member is ignored when the type is `AT_NULL`. Let's stop relying
on an unspecified value.

For others who want to check, see page 37 and 38:
https://gitlab.com/x86-psABIs/x86-64-ABI/-/wikis/uploads/221b09355dd540efcbe61b783b6c0ece/x86-64-psABI-2023-09-26.pdf

> This is a matter of taste, but I think processing the data in a structured
> way by coercing it into an array of type value pairs, using multiple
> return style, and a for loop with a clear exit condition is more readable
> than the existing infinite loop with multiple exit points and a return
> value variable.

Ok. It's more readable using your way. One thing that bothers me a bit
is type of 'a_type'. On page 37, the ABI defines the auxv type-val pair
as:

  typedef struct
  {
    int a_type;
    union {
      long a_val;
      void *a_ptr;
      void (*a_fnc)();
    } a_un;
  } auxv_t;

Assuming the arch is x86-64 Linux. Note that 'a_type' is an 'int' which
is 4 bytes in size, but we use 'unsigned long' instead of 'int' to
represent it. However, since 'a_un' needs to be 8 bytes aligned, the
compiler will put a 4 bytes padding between 'a_type' and 'a_un', so it
ends up just fine (on x86-64).

What do you think about other architectures? Will it potentially be
misinterpreted?

I tried to compare it in my head for i386, but it also ends up just
fine. I don't know other architectures.

> I also added a call to set errno to ENOENT when the entry is not found as
> glibc does which allows programs to disambiguate between the case of an
> auxv that is not present, and one that is, but with value zero.

Good catch!

-- 
Ammar Faizi