Messages in this thread | | | From | Andy Lutomirski <> | Date | Fri, 13 Dec 2019 09:49:28 -0800 | Subject | Re: [PATCH] x86-64/entry: add instruction suffix to SYSRET |
| |
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
| |