fstyp: detect Raspberry Pi Pico boot filesystem as FAT

fstyp looks for a 0x55 0xAA signature at offset 510, but this is not
required by specifications and is not proivded by the Raspberry Pi Pico
bootloader.

We should really remove the signature check and implement a more
comprehensive BPB validation instead, but it will require more
investigation and testing.  For now just add a special case for the
Raspberry Pi Pico bootloader, to avoid introducing regressions or new
false positives.

PR:		262896
Reviewed by:	delphij
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D34699

(cherry picked from commit 868c1b8431f297ade8deba5baf903f73cf5e11c6)
(cherry picked from commit 27c2f016b86744aa5d4c6031b4ef2fc16bbf6546)

1 files changed, 19 insertions, 2 deletions

diff --git a/usr.sbin/fstyp/msdosfs.c b/usr.sbin/fstyp/msdosfs.c
index 3d86802f6a2e..2e74f769ca97 100644
--- a/usr.sbin/fstyp/msdosfs.c
+++ b/usr.sbin/fstyp/msdosfs.c
@@ -41,6 +41,24 @@ __FBSDID("$FreeBSD$");
 
 #define LABEL_NO_NAME		"NO NAME    "
 
+/*
+ * XXX the signature 0x55 0xAA as the last two bytes of 512 is not required
+ * by specifications, but was historically required by fstyp.  This check
+ * should be removed, with a more comprehensive BPB validation instead.
+ */
+static bool
+check_signature(uint8_t sector0[512])
+{
+	/* Check for the FAT boot sector signature. */
+	if (sector0[510] == 0x55 && sector0[511] == 0xaa)
+		return (true);
+	/* Special case for Raspberry Pi Pico bootloader. */
+	if (sector0[510] == 0 && sector0[511] == 0 &&
+	    sector0[0] == 0xeb && sector0[1] == 0x3c && sector0[2] == 0x90)
+		return (true);
+	return (false);
+}
+
 int
 fstyp_msdosfs(FILE *fp, char *label, size_t size)
 {
@@ -57,8 +75,7 @@ fstyp_msdosfs(FILE *fp, char *label, size_t size)
 	if (sector0 == NULL)
 		return (1);
 
-	/* Check for the FAT boot sector signature. */
-	if (sector0[510] != 0x55 || sector0[511] != 0xaa) {
+	if (!check_signature(sector0)) {
 		goto error;
 	}