Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 12 Dec 2019 13:43:12 -0800 | Subject | Re: [PATCH] x86-64/entry: add instruction suffix to SYSRET |
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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?
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.
--Andy
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