Messages in this thread | | | Subject | Re: [PATCH] x86-64/entry: add instruction suffix to SYSRET | From | Jan Beulich <> | Date | Fri, 13 Dec 2019 10:55:31 +0100 |
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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).
> Anyway, the change itself is Acked-by: Andy Lutomirski <luto@kernel.org> > > But let's please clarify the changelog: > > ignore_sysret contains an unsuffixed 'sysret' instruction. gas > correctly interprets this as sysretl, but leaving it up to gas to > guess when there is no register operand that implies a size is bad > practice, and upstream gas is likely to warn about this in the future. > Use 'sysretl' explicitly. This does not change the assembled output.
Fine with me, changed.
Jan
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