Re: [PATCH 1/2] x86/fred: Reconstruct the #GP context for rejected INT instructions

["H. Peter Anvin" <hpa@zytor.com>]

On 2026-09-17 16:09, Matthew Schwartz wrote:
> FRED event delivery does not use the IDT, so the gate DPL check that
> rejects a user INT n falls to software (Intel FRED specification [1],
> section 8.3). fred_intx() rejects the same vectors as IDT delivery, but
> reports a zero error code and the IP after the INT. This breaks the
> signal ABI. Wine uses the error code to recognize INT 0x2d, so the
> changed context turns a handled breakpoint into an access violation in
> Elden Ring.
> 
> Rewind IP using the instruction length in the augmented SS and
> synthesize the IDT selector error code, (vector << 3) | 2. Set RF in the
> saved flags, as the CPU does for a #GP fault. Section 5.2.1 defines the
> saved vector, instruction length and RF state. The supplied length
> handles prefixes without reading user memory. Limit the changes to
> already-rejected software interrupts, preserving the accepted INT3, INT4
> and enabled INT80 paths and hardware exceptions. With IA32 emulation
> disabled, INT 0x80 now reports the same #GP as the DPL 0 gate IDT
> installs there. The rewound IP also stops fixup_iopl_exception() from
> inspecting the byte after the INT.
> 
> Also clear the software event flag. Section 6.2.3 specifies that ERETU
> with this flag and TF set traps before executing any user instruction. A
> tracer that suppresses SIGSEGV and resumes with TF set expects the next
> instruction to run first, as after IRET. The sigreturn path clears the
> same flag for this reason in prevent_single_step_upon_eretu().
> 
> [1] Intel Flexible Return and Event Delivery (FRED) Specification,
> revision 9.0 (346446-009US), sections 5.2.1, 6.2.3 and 8.3.
> 
> Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code")
> Reported-by: Paul Gofman <pgofman@codeweavers.com>
> Closes: https://gitlab.freedesktop.org/mesa/mesa/-/work_items/15745
> Closes: https://gitlab.freedesktop.org/mesa/mesa/-/work_items/16132
> Signed-off-by: Matthew Schwartz <matthew.schwartz@linux.dev>
> Link: https://cdrdv2.intel.com/v1/dl/getContent/678938 # [1]

Reviewed-by: H. Peter Anvin <hpa@zytor.com>

This really should go into -stable.

	-hpa