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FAT: allow 0xE9 near jump in fat_read_static_bpb()

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fat_read_static_bpb() only accepts a short jump as a valid bootstrap code
signature for DOS 1.x volumes when the dos1xfloppy mount option is used.

However, according to the Microsoft fatgen103.doc, the BS_jmpBoot field
has two allowed forms: 0xEB 0x??  0x90 (short jump + NOP) and 0xE9 0x?? 
0x??  (near jump).  The specification explicitly states that either form
is acceptable.

This patch relaxes the check to also accept 0xE9 as the first byte of the
jump instruction.

Link: https://lore.kernel.org/20260412070109.5197-1-zhangcoder@yeah.net
Signed-off-by: Ziran Zhang <zhangcoder@yeah.net>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Ziran Zhang
2026-04-12 14:59:41 +08:00
committed by Andrew Morton
parent 0a2418c327
commit bc96ad381f
1 changed files with 3 additions and 2 deletions
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5
fs/fat/inode.c
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@@ -1479,8 +1479,9 @@ static int fat_read_static_bpb(struct super_block *sb,
int error = -EINVAL;
unsigned i;
/* 16-bit DOS 1.x reliably wrote bootstrap short-jmp code */
if (b->ignored[0] != 0xeb || b->ignored[2] != 0x90) {
/* 16-bit DOS 1.x reliably wrote bootstrap short-jmp or near-jmp code */
if ((b->ignored[0] != 0xeb || b->ignored[2] != 0x90) &&
(b->ignored[0] != 0xe9)) {
if (!silent)
fat_msg(sb, KERN_ERR,
"%s; no bootstrapping code", notdos1x);