linux-kernel - Re: [PATCH] fortify: turn strlen() into an inline function using __builtin_constant

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Message-ID: <20250111165844.6d95600f@pumpkin>
Date: Sat, 11 Jan 2025 16:58:44 +0000
From: David Laight <david.laight.linux@...il.com>
To: Vincent Mailhol <mailhol.vincent@...adoo.fr>
Cc: Kees Cook <kees@...nel.org>, Nathan Chancellor <nathan@...nel.org>, Nick
 Desaulniers <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>,
 Justin Stitt <justinstitt@...gle.com>, David Laight
 <david.laight@...lab.com>, linux-hardening@...r.kernel.org,
 linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] fortify: turn strlen() into an inline function using
 __builtin_constant_p()

On Sat, 11 Jan 2025 23:40:41 +0900
Vincent Mailhol <mailhol.vincent@...adoo.fr> wrote:

> On Thu. 9 Jan 2025 at 16:52, Vincent Mailhol <mailhol.vincent@...adoo.fr> wrote:
...
> Actually, I did more investigation and it is working for some strange
> reasons. Whenever the argument of a function named strlen() is a
> compile time constant, the compiler (both GCC and clang) will replace
> it with the string length on the argument, even if strlen() is
> programmed to return something else:
> 
>   https://godbolt.org/z/nK4b3fnM7
> 
> So it is only working because the compiler uses its builtin strlen()
> instead of the function we provided.

It depends on whether -ffreestanding is set.
If not set gcc/clang assume a lot of the basic libc functions have their expected
behaviour.

	David