sysfs: crash debugging

Print the name of the last-accessed sysfs file when we oops, to help track
down oopses which occur in sysfs store/read handlers. Because these oopses
tend to not leave any trace of the offending code in the stack traces.

Cc: Kay Sievers <[e-mail]>
Cc: Mathieu Desnoyers <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

arch/x86/kernel/dumpstack_32.c

···
 #include <linux/kexec.h>
 #include <linux/bug.h>
 #include <linux/nmi.h>
+#include <linux/sysfs.h>
 
 #include <asm/stacktrace.h>
 
···
 	printk("DEBUG_PAGEALLOC");
 #endif
 	printk("\n");
+	sysfs_printk_last_file();
 	if (notify_die(DIE_OOPS, str, regs, err,
 			current->thread.trap_no, SIGSEGV) == NOTIFY_STOP)
 		return 1;

arch/x86/kernel/dumpstack_64.c

···
 #include <linux/kexec.h>
 #include <linux/bug.h>
 #include <linux/nmi.h>
+#include <linux/sysfs.h>
 
 #include <asm/stacktrace.h>
 
···
 	printk("DEBUG_PAGEALLOC");
 #endif
 	printk("\n");
+	sysfs_printk_last_file();
 	if (notify_die(DIE_OOPS, str, regs, err,
 			current->thread.trap_no, SIGSEGV) == NOTIFY_STOP)
 		return 1;

fs/sysfs/file.c

···
 #include <linux/poll.h>
 #include <linux/list.h>
 #include <linux/mutex.h>
+#include <linux/limits.h>
 #include <asm/uaccess.h>
 
 #include "sysfs.h"
+
+/* used in crash dumps to help with debugging */
+static char last_sysfs_file[PATH_MAX];
+void sysfs_printk_last_file(void)
+{
+	printk(KERN_EMERG "last sysfs file: %s\n", last_sysfs_file);
+}
 
 /*
  * There's one sysfs_buffer for each open file and one
···
 	struct sysfs_buffer *buffer;
 	struct sysfs_ops *ops;
 	int error = -EACCES;
+	char *p;
+
+	p = d_path(&file->f_path, last_sysfs_file, sizeof(last_sysfs_file));
+	if (p)
+		memmove(last_sysfs_file, p, strlen(p) + 1);
 
 	/* need attr_sd for attr and ops, its parent for kobj */
 	if (!sysfs_get_active_two(attr_sd))

include/linux/sysfs.h

···
 
 void sysfs_notify(struct kobject *kobj, char *dir, char *attr);
 
+void sysfs_printk_last_file(void);
+
 extern int __must_check sysfs_init(void);
 
 #else /* CONFIG_SYSFS */
···
 static inline int __must_check sysfs_init(void)
 {
 	return 0;
+}
+
+static inline void sysfs_printk_last_file(void)
+{
 }
 
 #endif /* CONFIG_SYSFS */