From: Andy Lutomirski <luto@kernel.org>
To: Jan Beulich <jbeulich@suse.com>
Cc: Andy Lutomirski <luto@kernel.org>,
"the arch/x86 maintainers" <x86@kernel.org>,
lkml <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86-64/entry: add instruction suffix to SYSRET
Date: Fri, 13 Dec 2019 09:49:28 -0800 [thread overview]
Message-ID: <CALCETrXoK+6gNn=3_yZdkHScd=N-a2f_VPC-svkFfHVsiVusVw@mail.gmail.com> (raw)
In-Reply-To: <ed9d8df6-0fe7-ca15-bab2-4d9cbbfe62f0@suse.com>
On Fri, Dec 13, 2019 at 1:55 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 12.12.2019 22:43, Andy Lutomirski wrote:
> > On Tue, Dec 10, 2019 at 7:40 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 10.12.2019 16:29, Andy Lutomirski wrote:
> >>>> On Dec 10, 2019, at 2:48 AM, Jan Beulich <JBeulich@suse.com> wrote:
> >>>>
> >>>> Omitting suffixes from instructions in AT&T mode is bad practice when
> >>>> operand size cannot be determined by the assembler from register
> >>>> operands, and is likely going to be warned about by upstream gas in the
> >>>> future. Add the missing suffix here.
> >>>>
> >>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> >>>>
> >>>> --- a/arch/x86/entry/entry_64.S
> >>>> +++ b/arch/x86/entry/entry_64.S
> >>>> @@ -1728,7 +1728,7 @@ END(nmi)
> >>>> SYM_CODE_START(ignore_sysret)
> >>>> UNWIND_HINT_EMPTY
> >>>> mov $-ENOSYS, %eax
> >>>> - sysret
> >>>> + sysretl
> >>>
> >>> Isn’t the default sysretq? sysretl looks more correct, but that suggests
> >>> that your changelog is wrong.
> >>
> >> No, this is different from ret, and more like iret and lret.
> >>
> >>> Is this code even reachable?
> >>
> >> Yes afaict, supported by the comment ahead of the symbol. syscall_init()
> >> puts its address into MSR_CSTAR when !IA32_EMULATION.
> >>
> >
> > What I meant was: can a program actually get itself into 32-bit mode
> > to execute a 32-bit SYSCALL instruction?
>
> Why not? It can set up a 32-bit code segment descriptor, far-branch
> into it, and then execute SYSCALL. I can't see anything preventing
> this in the logic involved in descriptor adjustment system calls. In
> fact it looks to be at least partly the opposite - fill_ldt()
> disallows creation of 64-bit code segments (oddly enough
> fill_user_desc() then still copies the bit back, despite there
> apparently being no way for it to get set).
Do we allow creation of 32-bit code segments on !IA32_EMULATION
kernels? I think we shouldn't, but I'm not really sure.
Anyway, this is irrelevant to the patch at hand.
--Andy
next prev parent reply other threads:[~2019-12-13 22:05 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-10 10:48 [PATCH] x86-64/entry: add instruction suffix to SYSRET Jan Beulich
2019-12-10 15:29 ` Andy Lutomirski
2019-12-10 15:40 ` Jan Beulich
2019-12-12 21:43 ` Andy Lutomirski
2019-12-13 9:55 ` Jan Beulich
2019-12-13 17:49 ` Andy Lutomirski [this message]
2019-12-16 10:11 ` Jan Beulich
2019-12-16 15:23 ` Brian Gerst
2019-12-19 2:39 ` Andy Lutomirski
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CALCETrXoK+6gNn=3_yZdkHScd=N-a2f_VPC-svkFfHVsiVusVw@mail.gmail.com' \
--to=luto@kernel.org \
--cc=jbeulich@suse.com \
--cc=linux-kernel@vger.kernel.org \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.