linux-kernel - [PATCH] module: fix reference to mod->percpu after freeing module.

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Message-Id: <201005311953.51694.rusty@rustcorp.com.au>
Date:	Mon, 31 May 2010 19:53:50 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Brandon Philips <brandon@...p.org>,
	Jon Masters <jonathan@...masters.org>,
	Tejun Heo <htejun@...il.com>,
	Masami Hiramatsu <mhiramat@...hat.com>
Subject: [PATCH] module: fix reference to mod->percpu after freeing module.

Rafael sees a sometimes crash at precpu_modfree from kernel/module.c; it
only occurred with another (since-reverted) patch, but that patch simply
changed timing to uncover this bug, it was otherwise unrelated.

The comment about the mod being freed is self-explanatory, but neither
Tejun nor I read it.  This bug was introduced in 259354deaa, after it
had previously been fixed in 6e2b75740b.  How embarrassing.

Reported-by: "Rafael J. Wysocki" <rjw@...k.pl>
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
Cc: Tejun Heo <tj@...nel.org>
Cc: Masami Hiramatsu <mhiramat@...hat.com>
Tested-by: "Rafael J. Wysocki" <rjw@...k.pl>
---
 kernel/module.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/module.c b/kernel/module.c
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2031,6 +2031,7 @@ static noinline struct module *load_modu
 	long err = 0;
 	void *ptr = NULL; /* Stops spurious gcc warning */
 	unsigned long symoffs, stroffs, *strmap;
+	void __percpu *percpu;
 
 	mm_segment_t old_fs;
 
@@ -2175,6 +2176,8 @@ static noinline struct module *load_modu
 			goto free_mod;
 		sechdrs[pcpuindex].sh_flags &= ~(unsigned long)SHF_ALLOC;
 	}
+	/* Keep this around for failure path. */
+	percpu = mod_percpu(mod);
 
 	/* Determine total sizes, and put offsets in sh_entsize.  For now
 	   this is done generically; there doesn't appear to be any
@@ -2480,7 +2483,7 @@ static noinline struct module *load_modu
 	module_free(mod, mod->module_core);
 	/* mod will be freed with core. Don't access it beyond this line! */
  free_percpu:
-	percpu_modfree(mod);
+	free_percpu(percpu);
  free_mod:
 	kfree(args);
 	kfree(strmap);
--
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