linux-kernel - Re: arch/x86/kernel/sys_x86_64.c: rationale for 0x40000000 for MAP

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Date: Sun, 11 Feb 2024 23:07:40 -0800
From: Andi Kleen <ak@...ux.intel.com>
To: Bagas Sanjaya <bagasdotme@...il.com>
Cc: hapter@...blaze.it, mingo@...hat.com, tglx@...utronix.de, bp@...en8.de,
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: arch/x86/kernel/sys_x86_64.c: rationale for 0x40000000 for
 MAP_32BIT's start address?

> >> Unfortunately this does not supply a rationale for starting from 0x40000000,
> >> which seems very arbitrary, and the git commit has been there since the
> >> beginning of time (i.e. as far the the git history goes), so the git blame
> >> has not helped much to clarify it. I was also not able to find who "AK" was.
> > 
> > That was from commit 717db2f9f36805 ("[PATCH] x86-64 updates for 2.5.54")
> > in tglx/history.git repo [1], authored by Andi Kleen. Cc'ing him.
> > 
> 
> [1]: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/arch/x86_64/kernel/sys_x86_64.c?id=717db2f9f36805d85c695771ea7d712812896aa7

I thought the comment was clear? The 1GB start is to avoid conflicts with the brk heap,
which grows up.

The flag is really obsolete, if you want limited relocations there are
better ways to do it that don't limit ASLR. 

It was originally because the custom module loader in X.org didn't support a PLT.


-Andi