Unchanged and unpatchable function moves from .text to .text.unlikely

[joe-lawrence]

Xinhua Li reports another potential bug similar to #1385 (building the following kpatch with RHEL9 kernel-5.14.0-362.24.1.el9_3.x86_64) except this time it's an UNchanged function that moves (so create-diff-object marks as CHANGED), but then it fails the mcount/fentry check as it's annotated as notrace:

--- src.orig/arch/x86/kernel/cpu/proc.c	2024-04-15 14:46:29.203617033 -0400
+++ src/arch/x86/kernel/cpu/proc.c	2024-04-15 14:46:43.803518044 -0400
@@ -65,7 +65,8 @@ static int show_cpuinfo(struct seq_file
 	int i;
 
 	cpu = c->cpu_index;
-	seq_printf(m, "processor\t: %u\n"
+	printk(KERN_ALERT "show_cpuinfo current \n");
+	seq_printf(m, "TAM demo processor\t: %u\n"
 		   "vendor_id\t: %s\n"
 		   "cpu family\t: %d\n"
 		   "model\t\t: %u\n"

results in kpatch-build error:

proc.ORIG.o: function cpumask_weight has no fentry/mcount call, unable to patch
ERROR: proc.ORIG.o: 1 function(s) can not be patched
create-diff-object: unreconcilable difference

[joe-lawrence]

repro-1386.tar.gz

Note that cpumask_weight() moves from .text.cpumask_weight to .text.unlikely.cpumask_weight, but otherwise carries no object code differences:

$ diff -upr \
      <(objdump -Dr -j.text.cpumask_weight          proc.ORIG.o) \
      <(objdump -Dr -j.text.unlikely.cpumask_weight proc.PATCHED.o)
--- /dev/fd/63  2024-04-15 14:53:36.281721258 -0400
+++ /dev/fd/62  2024-04-15 14:53:36.282721251 -0400
@@ -1,8 +1,8 @@
 
-proc.ORIG.o:     file format elf64-x86-64
+proc.PATCHED.o:     file format elf64-x86-64
 
 
-Disassembly of section .text.cpumask_weight:
+Disassembly of section .text.unlikely.cpumask_weight:
 
 0000000000000000 <__pfx_cpumask_weight>:
    0:  90                      nop

cpumask_weight() is included because create-diff-object considers the section change as allowed and marks the symbol as CHANGED:

static void kpatch_compare_correlated_symbol(struct symbol *sym)
{

	[ ... snip ... ]

	/*
	 * If two symbols are correlated but their sections are not, then the
	 * symbol has changed sections.  This is only allowed if the symbol is
	 * moving out of an ignored section, or moving between normal/hot/unlikely
	 * subsections.
	 */
	if (sym1->sec && sym2->sec && sym1->sec->twin != sym2->sec) {
		if ((sym2->sec->twin && sym2->sec->twin->ignore) ||
		    kpatch_subsection_changed(sym1->sec, sym2->sec))
			sym->status = CHANGED;
		else
			DIFF_FATAL("symbol changed sections: %s", sym1->name);
	}

	[ ... snip ... ]

The CHANGED status means that we will be trying to kpatch the original function with the new one.

[github-actions]

This issue has been open for 30 days with no activity and no assignee. It will be closed in 7 days unless a comment is added.

[github-actions]

This issue was closed because it was inactive for 7 days after being marked stale.

[github-actions]

This issue has been open for 30 days with no activity and no assignee. It will be closed in 7 days unless a comment is added.

[github-actions]

This issue was closed because it was inactive for 7 days after being marked stale.