linux-kernel - Re: [PATCH] nolibc: optimise _start() on x86

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Date:   Sat, 2 Dec 2023 14:23:59 +0100
From:   Willy Tarreau <w@....eu>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     Thomas Weißschuh <linux@...ssschuh.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nolibc: optimise _start() on x86_64

Hi Alexey,

On Sat, Dec 02, 2023 at 03:45:13PM +0300, Alexey Dobriyan wrote:
> Just jump into _start_c, it is not going to return anyway.

Thanks, but what's upper in the stack there ? I'm trying to make sure
that if _start_c returns we don't get a random behavior. If we get a
systematic crash (e.g. 0 always there) that's fine, what would be
annoying would be random infinite loops etc. In the psABI description
(table 3.9) I'm seeing "undefined" before argc, which I don't find
much appealing.

> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
> ---
> 
> 	Also, kernel clears all registers before starting process,
> 	I'm not sure why
> 
> 		xor ebp, ebp
> 
> 	was added.

Hmmm psABI says:

  Only the registers listed below have specied values at process entry:

  %rbp The content of this register is unspecied at process initialization
       time, but the user code should mark the deepest stack frame by setting
       the frame pointer to zero.

  %rsp The stack pointer holds the address of the byte with lowest address
	which is part of the stack. It is guaranteed to be 16-byte aligned at
        process entry.

  %rdx a function pointer that the application should register with atexit (BA_OS).

Thus apparently it's documented as being our job to clear it :-/

Willy