Re: [PATCH] Revert "x86/fpu: Refine and simplify the magic number check during signal return"

[Chang S. Bae]

On 4/28/2026 5:06 PM, Andrei Vagin wrote:
> The reverted commit broke applications that construct signal frames in
> userspace (such as CRIU and gVisor) if the frame's xstate size is
> smaller than the kernel's fpstate->user_size.

In the extended state area, the sigframe embeds the hardware-defined 
XSAVE format. If CPU A and CPU B support different XSTATE features, the 
layout (size and offsets) differ across systems. However, within a 
system, the layout is invariant. Userspace can query CPUID to obtain the 
exact offset and sizes, which effectively defines the ABI.

On top of the XSAVE data, the kernel appends metadata (e.g. the xstate 
size and magic values). In particular fpstate->user_size is written by 
save_sw_bytes() at signal delivery. On sigreturn, the kernel validates 
this, which is a symmetric and straightforward check.

Because the format is hardware-defined, arbitrary size mismatches should 
not be allowed. The sigframe should match the CPU-defined XSAVE layout. 
So the change in fact strengthens the sanity check.

> Furthermore, this introduces a critical issue for checkpoint/restore
> tools like CRIU. If a process is checkpointed while inside a signal
> handler, its stack contains a signal frame formatted according to the
> source host's xstate capabilities.  If that process is later restored on
> a destination host with larger xstate capabilities (e.g., a newer CPU
> with more features enabled, resulting in a larger fpstate->user_size),
> the kernel will look for FP_XSTATE_MAGIC2 at the destination host's
> larger user_size offset instead of the offset encoded in the frame's
> fx_sw->xstate_size.  This causes the magic2 check to fail, forcing
> sigreturn to silently fall back to "FX-only" mode.

It seems that userspace could translate the XSAVE buffer from CPU A's 
format to CPU B's format during restore. If so, the frame can be 
consistent with the destination system without modifying 
fx_sw->xstate_size, and the kernel-side validation would continue to 
work as intended.

Thanks,
Chang