Re: [PATCH v2] powerpc/32: remove bogus ppc_select syscall

[Christophe Leroy]

Le 04/03/2021 à 16:17, Arnd Bergmann a écrit :
> On Thu, Mar 4, 2021 at 1:51 PM Christophe Leroy
> <christophe.leroy@xxxxxxxxxx> wrote:
> > From: Arnd Bergmann <arnd@xxxxxxxx>
> >
> > 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@xxxxxxxxxxx>, 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@xxxxxxxx>
> > Cc: Halesh Sadashiiv <halesh.sadashiv@xxxxxxxxxxx>
> > [chleroy: Rebased and updated the number of years elapsed in the commit message]
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> > ---
> > First version was in 2008, at that time it was rejected, see
> > http://patchwork.ozlabs.org/project/linuxppc-dev/patch/200809240839.14902.arnd@xxxxxxxx/
>
> 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