[PATCH 4/7] x86/fpu: Document reasoning of FX-only fallback

[Andrei Vagin <avagin@google.com>]

Add a comment to check_xstate_in_sigframe() to explain the reasoning behind
falling back to the FX-only state when signal frame metadata is
inconsistent.

The fallback is intended to preserve backward compatibility with legacy
user-space processes that are not aware of XSAVE states and might only
fill or copy the legacy FP state.

However, this fallback should be avoided whenever possible. If a process
was actively using extended features, falling back to the FX-only state
silently resets those extended registers to their initial state, which
can lead to silent user-space state corruption.

Reviewed-by: Alexander Mikhalitsyn <alexander@mihalicyn.com>
Reviewed-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Andrei Vagin <avagin@google.com>
---
 arch/x86/kernel/fpu/signal.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index b9dcd0cd0e41..d56819fe491e 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -54,6 +54,14 @@ static inline bool check_xstate_in_sigframe(struct fxregs_state __user *buf_fx,
 	if (likely(magic2 == FP_XSTATE_MAGIC2))
 		return true;
 err_setfx:
+	/*
+	 * The fallback to FX-only state is used to preserve backward
+	 * compatibility with user-space processes that are not aware of xsave
+	 * states.
+	 *
+	 * In all other cases, returning false (to trigger SIGSEGV) is
+	 * preferred to avoid silent user-space state corruption.
+	 */
 	trace_x86_fpu_xstate_check_failed(x86_task_fpu(current));
 
 	/* Set the parameters for fx only state */
-- 
2.56.0.rc1.310.g51773c2048-goog