linux-kernel - [PATCH RFC] powerpc: Panic on jump label code patching failure

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Message-ID: <20250905061135.1451362-1-ajd@linux.ibm.com>
Date: Fri,  5 Sep 2025 16:11:35 +1000
From: Andrew Donnellan <ajd@...ux.ibm.com>
To: linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        maddy@...ux.ibm.com, mpe@...erman.id.au, christophe.leroy@...roup.eu,
        peterz@...radead.org, jpoimboe@...nel.org, jbaron@...mai.com
Cc: npiggin@...il.com, rostedt@...dmis.org, ardb@...nel.org,
        Erhard Furtner <erhard_f@...lbox.org>
Subject: [PATCH RFC] powerpc: Panic on jump label code patching failure

If patch_branch() or patch_instruction() fails while updating a jump
label, we presently fail silently, leading to unpredictable behaviour
later on.

Change arch_jump_label_transform() to panic on a code patching failure,
matching the existing behaviour of arch_static_call_transform().

Reported-by: Erhard Furtner <erhard_f@...lbox.org>
Signed-off-by: Andrew Donnellan <ajd@...ux.ibm.com>

---

Ran into this while debugging an issue that Erhard reported to me about my
PAGE_TABLE_CHECK series on a G4, where updating a static key failed
silently, but only for one call site, leading to an incorrect reference
count later on. This looks to be due to the issue fixed in [0]. A loud
failure would have saved us all considerable debugging time.

Should I change the return type of arch_jump_label_transform() and handle
this in an arch-independent way? Are there other users of code patching
in powerpc that ought to be hardened?

Or is this excessive?

[0] https://patchwork.ozlabs.org/project/linuxppc-dev/patch/4b5e6eb281d7b1ea77619bee17095f905a125168.1757003584.git.christophe.leroy@csgroup.eu/
---
 arch/powerpc/kernel/jump_label.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/jump_label.c b/arch/powerpc/kernel/jump_label.c
index 2659e1ac8604..80d41ed7ac50 100644
--- a/arch/powerpc/kernel/jump_label.c
+++ b/arch/powerpc/kernel/jump_label.c
@@ -12,9 +12,14 @@ void arch_jump_label_transform(struct jump_entry *entry,
 			       enum jump_label_type type)
 {
 	u32 *addr = (u32 *)jump_entry_code(entry);
+	int err;

 	if (type == JUMP_LABEL_JMP)
-		patch_branch(addr, jump_entry_target(entry), 0);
+		err = patch_branch(addr, jump_entry_target(entry), 0);
 	else
-		patch_instruction(addr, ppc_inst(PPC_RAW_NOP()));
+		err = patch_instruction(addr, ppc_inst(PPC_RAW_NOP()));
+
+	if (err)
+		panic("%s: patching failed, err %d, type %d, addr %pS, target %pS\n",
+		      __func__, err, type, addr, (void *)jump_entry_target(entry));
 }
-- 
2.51.0