Messages in this thread |  | | From | Brian Gerst <> | Date | Mon, 16 Dec 2019 10:23:15 -0500 | Subject | Re: [PATCH] x86-64/entry: add instruction suffix to SYSRET |
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On Mon, Dec 16, 2019 at 5:12 AM Jan Beulich <jbeulich@suse.com> wrote: > > On 13.12.2019 18:49, Andy Lutomirski wrote: > > 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? > > As per above - I think so. > > > I think we shouldn't, but I'm not really sure. > > It may be a little exotic, but I can't see any reason to disallow > a 64-bit process to switch to compatibility mode temporarily. One > contrived use case could be to be able to invoke INTO or BOUND.
I think it should be kept intact for future use by WINE. WINE is currently set up so that 32/16-bit Windows emulation needs a 32-bit build against 32-bit Linux libraries, using the kernel compat layer. With many distributions wanting to drop 32-bit support this has been a big sticking point. If WINE could be modified so that the core is always built as 64-bit with 32-bit compatibility handled entirely in userspace, that would remove its dependency on 32-bit Linux libraries and thus wouldn't require IA32_EMULATION.
-- Brian Gerst
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