Messages in this thread | | | Date | Fri, 20 Jan 2023 12:50:51 -0800 | From | "H. Peter Anvin" <> | Subject | Re: the x86 sysret_rip test fails on the Intel FRED architecture |
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On January 20, 2023 10:52:02 AM PST, Andrew Cooper <Andrew.Cooper3@citrix.com> wrote: >On 20/01/2023 5:45 pm, Dave Hansen wrote: >> On 1/19/23 23:49, Li, Xin3 wrote: >>> The x86 sysret_rip test has the following assertion: >>> >>> /* R11 and EFLAGS should already match. */ >>> assert(ctx->uc_mcontext.gregs[REG_EFL] == >>> ctx->uc_mcontext.gregs[REG_R11]); >>> >>> This is being tested to avoid kernel state leak due to sysret vs iret, >>> but that on FRED r11 is *always* preserved, and the test just fails. >> Let's figure out the reason that FRED acts differently, first. Right >> now, the SDM says: >> >> SYSCALL also saves RFLAGS into R11 >> >> so that behavior of SYSCALL _looks_ architectural to me. Was this >> change in SYSCALL behavior with FRED intentional? > >FRED 3.0 Section 7.4 says the only changes for the SYSCALL and SYSENTER >instructions are the enablement conditions. Nowhere else is there >mention of a FRED OS needing to emulate legacy syscall behaviour by >adjusting %r11/%rcx > >However, ERETU does handle flags different to SYSRET (in particular, I >think you can establish TF on the instruction boundary after SYSCALL >now). What are the raw values of REG_EFL and REG_R11 ? > >~Andrew >
Just to avoid any confusion:
Syscall and sysenter in a FRED system are treated equivalently to software interrupts, e.g. INT 0x80. They do not modify any registers.
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