git: 1318b1ee9c4e - stable/12 - fstyp: detect Raspberry Pi Pico boot filesystem as FAT

The branch stable/12 has been updated by emaste:

URL: https://cgit.FreeBSD.org/src/commit/?id=1318b1ee9c4ee9ef6c1be600adc252a713b93961

commit 1318b1ee9c4ee9ef6c1be600adc252a713b93961
Author:     Ed Maste <emaste@FreeBSD.org>
AuthorDate: 2022-03-28 21:03:10 +0000
Commit:     Ed Maste <emaste@FreeBSD.org>
CommitDate: 2022-03-31 21:00:47 +0000

    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)
    (cherry picked from commit e06ce938ddc0222c6ed10a70108f71685923bd45)
---
 usr.sbin/fstyp/msdosfs.c | 21 +++++++++++++++++++--
 1 file 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;
 	}