linux-kernel - Re: [PATCH v2] powerpc/32: remove bogus ppc

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Message-ID: <aca143f6-fcfc-d89f-bb00-26e90257fbf6@csgroup.eu>
Date:   Thu, 4 Mar 2021 16:24:46 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        halesh.sadashiv@...sony.com,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] powerpc/32: remove bogus ppc_select syscall



Le 04/03/2021 à 16:17, Arnd Bergmann a écrit :
> On Thu, Mar 4, 2021 at 1:51 PM Christophe Leroy
> <christophe.leroy@...roup.eu> wrote:
>>
>> From: Arnd Bergmann <arnd@...db.de>
>>
>> The ppc_select function was introduced in linux-2.3.48 in order to support
>> code confusing the legacy select() calling convention with the standard one.
>> Even 24 years ago, all correctly built code should not have done this and
>> could have easily been phased out. Nothing that was compiled later should
>> actually try to use the old_select interface, and it would have been broken
>> already on all ppc64 kernels with the syscall emulation layer.
>>
>> This patch brings the 32 bit compat ABI and the native 32 bit ABI for
>> powerpc into a consistent state, by removing support for both the
>> old_select system call number and the handler for it.
>>
>> The bug report triggering this came from
>> Halesh Sadashiiv <halesh.sadashiv@...sony.com>, who discovered that the
>> 32 bit implementation of ppc_select would in case of a negative number
>> of file descriptors incorrectly return -EFAULT instead of -EINVAL.
>> There seems to be no way to fix this problem in a way that would
>> keep broken pre-1997 binaries running.
>>
>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>> Cc: Halesh Sadashiiv <halesh.sadashiv@...sony.com>
>> [chleroy: Rebased and updated the number of years elapsed in the commit message]
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>> ---
>> First version was in 2008, at that time it was rejected, see
>> http://patchwork.ozlabs.org/project/linuxppc-dev/patch/200809240839.14902.arnd@arndb.de/
> 
> The patch from 2008 did two things:
> 
> - it removed the ppc32 specific 'select' syscall at #82
> - it fixed the generic '_newselect' syscall at #142
> 
> Back then, the decision was to only address the second issue, which
> got merged in commit dad2f2fb0fc7 ("powerpc: Fix wrong error code from
> ppc32 select syscall").
> 
> It is probably ok to remove the old select system call now, but
> my changelog text no longer makes sense, as the patch has nothing
> to do with the bug that was reported back then.
> 

I understood that the original reported bug was that calling that version of select() with a 
negative value as first parametre would lead to a -EFAULT instead of -EINVAL. That's exactly the 
case here, if you set n = -1 you get into this (unsigned long)n > 4096 then the buffer is at 
0xffffffff and access_ok() won't grand access to it so the return value will be -EFAULT instead of 
-EINVAL.

Am I missing something ?

Christophe