pstore: Change kmsg_bytes storage size to u32

The types around kmsg_bytes were inconsistent. The global was unsigned
long, the argument to pstore_set_kmsg_bytes() was int, and the filesystem
option was u32. Given other internal limits, there's not much sense
in making a single pstore record larger than INT_MAX and it can't be
negative, so use u32 everywhere. Additionally, use READ/WRITE_ONCE and a
local variable in pstore_dump() to avoid kmsg_bytes changing during a
dump.

Link: https://lore.kernel.org/r/20250206191655.work.798-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

Author: kees

fs/pstore/inode.c

@@ -265,7 +265,7 @@ static void parse_options(char *options)
 static int pstore_show_options(struct seq_file *m, struct dentry *root)
 {
 	if (kmsg_bytes != CONFIG_PSTORE_DEFAULT_KMSG_BYTES)
-		seq_printf(m, ",kmsg_bytes=%lu", kmsg_bytes);
+		seq_printf(m, ",kmsg_bytes=%u", kmsg_bytes);
 	return 0;
 }
 

fs/pstore/internal.h

@@ -6,7 +6,7 @@
 #include <linux/time.h>
 #include <linux/pstore.h>
 
-extern unsigned long kmsg_bytes;
+extern unsigned int kmsg_bytes;
 
 #ifdef CONFIG_PSTORE_FTRACE
 extern void pstore_register_ftrace(void);
@@ -35,7 +35,7 @@ static inline void pstore_unregister_pmsg(void) {}
 
 extern struct pstore_info *psinfo;
 
-extern void	pstore_set_kmsg_bytes(int);
+extern void	pstore_set_kmsg_bytes(unsigned int bytes);
 extern void	pstore_get_records(int);
 extern void	pstore_get_backend_records(struct pstore_info *psi,
 					   struct dentry *root, int quiet);

fs/pstore/platform.c

@@ -92,8 +92,8 @@ module_param(compress, charp, 0444);
 MODULE_PARM_DESC(compress, "compression to use");
 
 /* How much of the kernel log to snapshot */
-unsigned long kmsg_bytes = CONFIG_PSTORE_DEFAULT_KMSG_BYTES;
-module_param(kmsg_bytes, ulong, 0444);
+unsigned int kmsg_bytes = CONFIG_PSTORE_DEFAULT_KMSG_BYTES;
+module_param(kmsg_bytes, uint, 0444);
 MODULE_PARM_DESC(kmsg_bytes, "amount of kernel log to snapshot (in bytes)");
 
 static void *compress_workspace;
@@ -107,9 +107,9 @@ static void *compress_workspace;
 static char *big_oops_buf;
 static size_t max_compressed_size;
 
-void pstore_set_kmsg_bytes(int bytes)
+void pstore_set_kmsg_bytes(unsigned int bytes)
 {
-	kmsg_bytes = bytes;
+	WRITE_ONCE(kmsg_bytes, bytes);
 }
 
 /* Tag each group of saved records with a sequence number */
@@ -278,6 +278,7 @@ static void pstore_dump(struct kmsg_dumper *dumper,
 			struct kmsg_dump_detail *detail)
 {
 	struct kmsg_dump_iter iter;
+	unsigned int	remaining = READ_ONCE(kmsg_bytes);
 	unsigned long	total = 0;
 	const char	*why;
 	unsigned int	part = 1;
@@ -300,7 +301,7 @@ static void pstore_dump(struct kmsg_dumper *dumper,
 	kmsg_dump_rewind(&iter);
 
 	oopscount++;
-	while (total < kmsg_bytes) {
+	while (total < remaining) {
 		char *dst;
 		size_t dst_size;
 		int header_size;