linux-kernel - Re: [PATCH] alpha: Fix personality flag propagation across an exec

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Message-Id: <24f03227-1b55-4e50-b6e9-7ac74fda2602@app.fastmail.com>
Date: Thu, 09 Jan 2025 09:01:16 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "John Paul Adrian Glaubitz" <glaubitz@...sik.fu-berlin.de>,
 "Richard Henderson" <richard.henderson@...aro.org>,
 "Matt Turner" <mattst88@...il.com>,
 "Eric W. Biederman" <ebiederm@...ssion.com>, "Kees Cook" <kees@...nel.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, linux-alpha@...r.kernel.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc: "Michael Cree" <mcree@...on.net.nz>, "Sam James" <sam@...too.org>,
 "Maciej W. Rozycki" <macro@...am.me.uk>,
 "Geert Uytterhoeven" <geert@...ux-m68k.org>,
 "Michael Karcher" <kernel@...rcher.dialup.fu-berlin.de>,
 "Chris Hofstaedtler" <zeha@...ian.org>, util-linux@...r.kernel.org
Subject: Re: [PATCH] alpha: Fix personality flag propagation across an exec

On Fri, Jan 3, 2025, at 15:01, John Paul Adrian Glaubitz wrote:

> 
>  #define SET_PERSONALITY(EX)					\
> -	set_personality(((EX).e_flags & EF_ALPHA_32BIT)		\
> -	   ? PER_LINUX_32BIT : PER_LINUX)
> +	set_personality((((EX).e_flags & EF_ALPHA_32BIT)	\
> +	   ? PER_LINUX_32BIT : PER_LINUX) | (current->personality & (~PER_MASK)))

This looks wrong to me: since ADDR_LIMIT_32BIT is not part of
PER_MASK, executing a regular binary from a taso binary no longer
reverts back to the entire 64-bit address space.

It seems that the behavior on most other architectures changed in 2012
commit 16f3e95b3209 ("cross-arch: don't corrupt personality flags upon
exec()").

At the time, the same bug existed on mips, parisc and tile, but those
got fixed quickly.

There are two related bits I don't quite understand:

- Do we still care about EF_ALPHA_32BIT? I see that it gets set by
 "alpha-linux-ld.bfd --taso", but could not find any documentation
 on what that flag is actually good for. On all other architectures,
 the address space limit gets enforced through a per-thread setting
 like TIF_32BIT, not through the personality that gets inherited
 by the child processes.

- all architectures other than x86 mask out the lower byte. Why
  not that one?

       Arnd