[PATCH v2] fs: FAT: Add support for DOS 1.x formatted volumes

[PATCH v2] fs: FAT: Add support for DOS 1.x formatted volumes

[Conrad Meyer]

When possible, infer DOS 2.x BIOS Parameter Block from block device
geometry (for floppies and floppy images). Update in-memory only. We
only perform this update when the entire BPB region is zeroed, like
produced by DOS 1.x-era FORMAT (and other OEM variations on DOS).

Fixes kernel.org bug #42617.

BPB default values are inferred from media size and a table.[0] Media
size is assumed to be static for archaic FAT volumes. See also [1].

[0]: https://en.wikipedia.org/wiki/File_Allocation_Table#Exceptions
[1]: http://www.win.tue.nl/~aeb/linux/fs/fat/fat-1.html

Signed-off-by: Conrad Meyer <cse.cem@gmail.com>
---
Diff from v3.14-rc8.

Changes since v1:
  * Check for FAT bootstrap prefix (EB xx 90) to help avoid conflicting with
    other filesystems
  * Move default from a switch to a static table

Thanks!
---
 fs/fat/inode.c | 93 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 93 insertions(+)

diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index 854b578..c31fbdc 100644
--- a/fs/fat/inode.c
+++ b/fs/fat/inode.c
@@ -35,9 +35,47 @@
 #define CONFIG_FAT_DEFAULT_IOCHARSET	""
 #endif
 
+#define KB_IN_SECTORS 2
+
 static int fat_default_codepage = CONFIG_FAT_DEFAULT_CODEPAGE;
 static char fat_default_iocharset[] = CONFIG_FAT_DEFAULT_IOCHARSET;
 
+static struct fat_floppy_defaults {
+	unsigned nr_sectors;
+	unsigned sec_per_clus;
+	unsigned dir_entries;
+	unsigned media;
+	unsigned fat_length;
+} floppy_defaults[] = {
+{
+	.nr_sectors = 160 * KB_IN_SECTORS,
+	.sec_per_clus = 1,
+	.dir_entries = 64,
+	.media = 0xFE,
+	.fat_length = 1,
+},
+{
+	.nr_sectors = 180 * KB_IN_SECTORS,
+	.sec_per_clus = 1,
+	.dir_entries = 64,
+	.media = 0xFC,
+	.fat_length = 2,
+},
+{
+	.nr_sectors = 320 * KB_IN_SECTORS,
+	.sec_per_clus = 2,
+	.dir_entries = 112,
+	.media = 0xFF,
+	.fat_length = 1,
+},
+{
+	.nr_sectors = 360 * KB_IN_SECTORS,
+	.sec_per_clus = 2,
+	.dir_entries = 112,
+	.media = 0xFD,
+	.fat_length = 2,
+},
+{ 0 } };
 
 static int fat_add_cluster(struct inode *inode)
 {
@@ -1246,6 +1284,58 @@ static unsigned long calc_fat_clusters(struct super_block *sb)
 }
 
 /*
+ * If this FAT filesystem is archaic (lacking a BIOS Parameter Block, ca. DOS
+ * 1.x), fix it up in-place by creating a DOS 2.x BIOS Parameter Block from
+ * defaults for the media size.
+ */
+static void fat_update_archaic_boot_sector(struct super_block *sb,
+	struct fat_boot_sector *b)
+{
+	struct fat_floppy_defaults *di;
+	sector_t bd_sects;
+
+	/* 16-bit DOS 1.x reliably wrote bootstrap short-jmp code */
+	if (b->ignored[0] != 0xeb || b->ignored[2] != 0x90)
+		return;
+
+	/*
+	 * If any value in this region is non-zero, don't assume it is archaic
+	 * DOS.
+	 */
+	if (get_unaligned_le16(&b->sector_size) != 0 || b->sec_per_clus != 0 ||
+		b->reserved != 0 || b->fats != 0 ||
+		get_unaligned_le16(&b->dir_entries) != 0 ||
+		get_unaligned_le16(&b->sectors) != 0 || b->media != 0 ||
+		b->fat_length != 0 || b->secs_track != 0 || b->heads != 0 ||
+		b->secs_track != 0 || b->heads != 0)
+		return;
+
+	bd_sects = part_nr_sects_read(sb->s_bdev->bd_part);
+	for (di = floppy_defaults; di->nr_sectors; di++) {
+		if (di->nr_sectors == bd_sects)
+			break;
+	}
+	if (di->nr_sectors == 0) {
+		fat_msg(sb, KERN_WARNING,
+			"DOS volume lacks BPB and isn't a recognized floppy size (%ld sectors)",
+			(long)bd_sects);
+		return;
+	}
+
+	fat_msg(sb, KERN_INFO,
+		"Volume lacks BPB but looks like archaic DOS; assuming default BPB values");
+
+	b->sec_per_clus = di->sec_per_clus;
+	put_unaligned_le16(di->dir_entries, &b->dir_entries);
+	b->media = di->media;
+	b->fat_length = cpu_to_le16(di->fat_length);
+	put_unaligned_le16(SECTOR_SIZE, &b->sector_size);
+	b->reserved = cpu_to_le16(1);
+	b->fats = 2;
+	put_unaligned_le16(bd_sects, &b->sectors);
+}
+
+/*
  * Read the super block of an MS-DOS FS.
  */
 int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
@@ -1297,6 +1387,8 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
 	}
 
 	b = (struct fat_boot_sector *) bh->b_data;
+	fat_update_archaic_boot_sector(sb, b);
+
 	if (!b->reserved) {
 		if (!silent)
 			fat_msg(sb, KERN_ERR, "bogus number of reserved sectors");
@@ -1364,6 +1456,7 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
 			goto out_fail;
 		}
 		b = (struct fat_boot_sector *) bh->b_data;
+		fat_update_archaic_boot_sector(sb, b);
 	}
 
 	mutex_init(&sbi->s_lock);
-- 
1.9.0

Re: [PATCH v2] fs: FAT: Add support for DOS 1.x formatted volumes

[OGAWA Hirofumi]

Conrad Meyer <cemeyer@uw.edu> writes:

> +static void fat_update_archaic_boot_sector(struct super_block *sb,
> +	struct fat_boot_sector *b)
> +{
> +	struct fat_floppy_defaults *di;
> +	sector_t bd_sects;
> +
> +	/* 16-bit DOS 1.x reliably wrote bootstrap short-jmp code */
> +	if (b->ignored[0] != 0xeb || b->ignored[2] != 0x90)
> +		return;
> +
> +	/*
> +	 * If any value in this region is non-zero, don't assume it is archaic
> +	 * DOS.
> +	 */
> +	if (get_unaligned_le16(&b->sector_size) != 0 || b->sec_per_clus != 0 ||
> +		b->reserved != 0 || b->fats != 0 ||
> +		get_unaligned_le16(&b->dir_entries) != 0 ||
> +		get_unaligned_le16(&b->sectors) != 0 || b->media != 0 ||
> +		b->fat_length != 0 || b->secs_track != 0 || b->heads != 0 ||
> +		b->secs_track != 0 || b->heads != 0)
> +		return;

Probably, too weak detection to use by default. So, how about to use
mount option to enable this?

And only if user asked to enable explicitly by mount option, allow this
format.

> +	bd_sects = part_nr_sects_read(sb->s_bdev->bd_part);
> +	for (di = floppy_defaults; di->nr_sectors; di++) {
> +		if (di->nr_sectors == bd_sects)
> +			break;
> +	}
> +	if (di->nr_sectors == 0) {
> +		fat_msg(sb, KERN_WARNING,
> +			"DOS volume lacks BPB and isn't a recognized floppy size (%ld sectors)",
> +			(long)bd_sects);
> +		return;
> +	}
> +
> +	fat_msg(sb, KERN_INFO,
> +		"Volume lacks BPB but looks like archaic DOS; assuming default BPB values");
> +
> +	b->sec_per_clus = di->sec_per_clus;
> +	put_unaligned_le16(di->dir_entries, &b->dir_entries);
> +	b->media = di->media;
> +	b->fat_length = cpu_to_le16(di->fat_length);
> +	put_unaligned_le16(SECTOR_SIZE, &b->sector_size);
> +	b->reserved = cpu_to_le16(1);
> +	b->fats = 2;
> +	put_unaligned_le16(bd_sects, &b->sectors);
> +}
> +
> +/*
>   * Read the super block of an MS-DOS FS.
>   */
>  int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
> @@ -1297,6 +1387,8 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
>  	}
>  
>  	b = (struct fat_boot_sector *) bh->b_data;
> +	fat_update_archaic_boot_sector(sb, b);

This would be better to set sbi->* directly, not via modified BPB.

>  	if (!b->reserved) {
>  		if (!silent)
>  			fat_msg(sb, KERN_ERR, "bogus number of reserved sectors");
> @@ -1364,6 +1456,7 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
>  			goto out_fail;
>  		}
>  		b = (struct fat_boot_sector *) bh->b_data;
> +		fat_update_archaic_boot_sector(sb, b);

This doesn't need. If logical_sector_size is 512, this format doesn't work.
-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

Re: [PATCH v2] fs: FAT: Add support for DOS 1.x formatted volumes

[Conrad Meyer]

On Mon, 31 Mar 2014 23:07:32 +0900
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp> wrote:

> Conrad Meyer <cemeyer@uw.edu> writes:
> 
> > +static void fat_update_archaic_boot_sector(struct
> > super_block *sb,
> > +	struct fat_boot_sector *b)
> > +{
> > +	struct fat_floppy_defaults *di;
> > +	sector_t bd_sects;
> > +
> > +	/* 16-bit DOS 1.x reliably wrote bootstrap
> > short-jmp code */
> > +	if (b->ignored[0] != 0xeb || b->ignored[2] !=
> > 0x90)
> > +		return;
> > +
> > +	/*
> > +	 * If any value in this region is non-zero,
> > don't assume it is archaic
> > +	 * DOS.
> > +	 */
> > +	if (get_unaligned_le16(&b->sector_size) != 0 ||
> > b->sec_per_clus != 0 ||
> > +		b->reserved != 0 || b->fats != 0 ||
> > +		get_unaligned_le16(&b->dir_entries) != 0
> > ||
> > +		get_unaligned_le16(&b->sectors) != 0 ||
> > b->media != 0 ||
> > +		b->fat_length != 0 || b->secs_track != 0
> > || b->heads != 0 ||
> > +		b->secs_track != 0 || b->heads != 0)
> > +		return;
> 
> Probably, too weak detection to use by default. So, how
> about to use mount option to enable this?
> 
> And only if user asked to enable explicitly by mount
> option, allow this format.

Okay. Do you have a preference for option name? "archaicdos",
"dos1x", "guessbpb", other?

> > +	bd_sects =
> > part_nr_sects_read(sb->s_bdev->bd_part);
> > +	for (di = floppy_defaults; di->nr_sectors; di++)
> > {
> > +		if (di->nr_sectors == bd_sects)
> > +			break;
> > +	}
> > +	if (di->nr_sectors == 0) {
> > +		fat_msg(sb, KERN_WARNING,
> > +			"DOS volume lacks BPB and isn't
> > a recognized floppy size (%ld sectors)",
> > +			(long)bd_sects);
> > +		return;
> > +	}
> > +
> > +	fat_msg(sb, KERN_INFO,
> > +		"Volume lacks BPB but looks like archaic
> > DOS; assuming default BPB values"); +
> > +	b->sec_per_clus = di->sec_per_clus;
> > +	put_unaligned_le16(di->dir_entries,
> > &b->dir_entries);
> > +	b->media = di->media;
> > +	b->fat_length = cpu_to_le16(di->fat_length);
> > +	put_unaligned_le16(SECTOR_SIZE, &b->sector_size);
> > +	b->reserved = cpu_to_le16(1);
> > +	b->fats = 2;
> > +	put_unaligned_le16(bd_sects, &b->sectors);
> > +}
> > +
> > +/*
> >   * Read the super block of an MS-DOS FS.
> >   */
> >  int fat_fill_super(struct super_block *sb, void *data,
> > int silent, int isvfat, @@ -1297,6 +1387,8 @@ int
> > fat_fill_super(struct super_block *sb, void *data, int
> > silent, int isvfat, } 
> >  	b = (struct fat_boot_sector *) bh->b_data;
> > +	fat_update_archaic_boot_sector(sb, b);
> 
> This would be better to set sbi->* directly, not via
> modified BPB.

Hm, I thought so too, but there are lots of sbi-> fields and
I didn't want to duplicate any shared filling logic in a
different place. But I will make the change...

> 
> >  	if (!b->reserved) {
> >  		if (!silent)
> >  			fat_msg(sb, KERN_ERR, "bogus
> > number of reserved sectors"); @@ -1364,6 +1456,7 @@ int
> > fat_fill_super(struct super_block *sb, void *data, int
> > silent, int isvfat, goto out_fail; }
> >  		b = (struct fat_boot_sector *)
> > bh->b_data;
> > +		fat_update_archaic_boot_sector(sb, b);
> 
> This doesn't need. If logical_sector_size is 512, this
> format doesn't work.

Right. Okay, give me some time to revise a v3 patch.

Thanks,
Conrad

Re: [PATCH v2] fs: FAT: Add support for DOS 1.x formatted volumes

[Andrew Morton]

On Sat, 29 Mar 2014 12:10:35 -0700 Conrad Meyer <cemeyer@uw.edu> wrote:

> When possible, infer DOS 2.x BIOS Parameter Block from block device
> geometry (for floppies and floppy images). Update in-memory only. We
> only perform this update when the entire BPB region is zeroed, like
> produced by DOS 1.x-era FORMAT (and other OEM variations on DOS).
> 
> Fixes kernel.org bug #42617.
> 
> BPB default values are inferred from media size and a table.[0] Media
> size is assumed to be static for archaic FAT volumes. See also [1].
> 
> [0]: https://en.wikipedia.org/wiki/File_Allocation_Table#Exceptions
> [1]: http://www.win.tue.nl/~aeb/linux/fs/fat/fat-1.html
> 
> ...
>
> --- a/fs/fat/inode.c
> +++ b/fs/fat/inode.c
> @@ -35,9 +35,47 @@
>  #define CONFIG_FAT_DEFAULT_IOCHARSET	""
>  #endif
>  
> +#define KB_IN_SECTORS 2
> +
>  static int fat_default_codepage = CONFIG_FAT_DEFAULT_CODEPAGE;
>  static char fat_default_iocharset[] = CONFIG_FAT_DEFAULT_IOCHARSET;
>  
> +static struct fat_floppy_defaults {
> +	unsigned nr_sectors;
> +	unsigned sec_per_clus;
> +	unsigned dir_entries;
> +	unsigned media;
> +	unsigned fat_length;
> +} floppy_defaults[] = {
> +{
> +	.nr_sectors = 160 * KB_IN_SECTORS,
> +	.sec_per_clus = 1,
> +	.dir_entries = 64,
> +	.media = 0xFE,
> +	.fat_length = 1,
> +},
> +{
> +	.nr_sectors = 180 * KB_IN_SECTORS,
> +	.sec_per_clus = 1,
> +	.dir_entries = 64,
> +	.media = 0xFC,
> +	.fat_length = 2,
> +},
> +{
> +	.nr_sectors = 320 * KB_IN_SECTORS,
> +	.sec_per_clus = 2,
> +	.dir_entries = 112,
> +	.media = 0xFF,
> +	.fat_length = 1,
> +},
> +{
> +	.nr_sectors = 360 * KB_IN_SECTORS,
> +	.sec_per_clus = 2,
> +	.dir_entries = 112,
> +	.media = 0xFD,
> +	.fat_length = 2,
> +},
> +{ 0 } };

We don't really need this EOF element.

>  static int fat_add_cluster(struct inode *inode)
>  {
> @@ -1246,6 +1284,58 @@ static unsigned long calc_fat_clusters(struct super_block *sb)
>  }
>  
>  /*
> + * If this FAT filesystem is archaic (lacking a BIOS Parameter Block, ca. DOS
> + * 1.x), fix it up in-place by creating a DOS 2.x BIOS Parameter Block from
> + * defaults for the media size.
> + */
> +static void fat_update_archaic_boot_sector(struct super_block *sb,
> +	struct fat_boot_sector *b)
> +{
> +	struct fat_floppy_defaults *di;
> +	sector_t bd_sects;
> +
> +	/* 16-bit DOS 1.x reliably wrote bootstrap short-jmp code */
> +	if (b->ignored[0] != 0xeb || b->ignored[2] != 0x90)
> +		return;
> +
> +	/*
> +	 * If any value in this region is non-zero, don't assume it is archaic
> +	 * DOS.
> +	 */
> +	if (get_unaligned_le16(&b->sector_size) != 0 || b->sec_per_clus != 0 ||
> +		b->reserved != 0 || b->fats != 0 ||
> +		get_unaligned_le16(&b->dir_entries) != 0 ||
> +		get_unaligned_le16(&b->sectors) != 0 || b->media != 0 ||
> +		b->fat_length != 0 || b->secs_track != 0 || b->heads != 0 ||
> +		b->secs_track != 0 || b->heads != 0)

Impressive!

> +		return;
> +
> +	bd_sects = part_nr_sects_read(sb->s_bdev->bd_part);
> +	for (di = floppy_defaults; di->nr_sectors; di++) {

Can do something like

	for (di = floppy_defaults;
		di < floppy_defaults + ARRAY_SIZE(floppy_defaults); di++) {

> +		if (di->nr_sectors == bd_sects)
> +			break;
> +	}
> +	if (di->nr_sectors == 0) {
> +		fat_msg(sb, KERN_WARNING,
> +			"DOS volume lacks BPB and isn't a recognized floppy size (%ld sectors)",
> +			(long)bd_sects);

sector_t can be u64 on 32-bit so one should really use %Lu and cast to
u64.

>
> ...
>

Re: [PATCH v2] fs: FAT: Add support for DOS 1.x formatted volumes

[Conrad Meyer]

On Mon, Mar 31, 2014 at 3:13 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Sat, 29 Mar 2014 12:10:35 -0700 Conrad Meyer <cemeyer@uw.edu> wrote:
>
>> +     .nr_sectors = 360 * KB_IN_SECTORS,
>> +     .sec_per_clus = 2,
>> +     .dir_entries = 112,
>> +     .media = 0xFD,
>> +     .fat_length = 2,
>> +},
>> +{ 0 } };
>
> We don't really need this EOF element.

Ah, right, I forgot about ARRAY_SIZE. This is an old version of this
patch, see v3 here: https://lkml.org/lkml/2014/3/31/275

But the same criticism is valid there, too.

>> +     if (get_unaligned_le16(&b->sector_size) != 0 || b->sec_per_clus != 0 ||
>> +             b->reserved != 0 || b->fats != 0 ||
>> +             get_unaligned_le16(&b->dir_entries) != 0 ||
>> +             get_unaligned_le16(&b->sectors) != 0 || b->media != 0 ||
>> +             b->fat_length != 0 || b->secs_track != 0 || b->heads != 0 ||
>> +             b->secs_track != 0 || b->heads != 0)
>
> Impressive!

I aim to please. Not sure what would be better -- memcmp() part of the
struct to a zeroed array?

>
>> +             return;
>> +
>> +     bd_sects = part_nr_sects_read(sb->s_bdev->bd_part);
>> +     for (di = floppy_defaults; di->nr_sectors; di++) {
>
> Can do something like
>
>         for (di = floppy_defaults;
>                 di < floppy_defaults + ARRAY_SIZE(floppy_defaults); di++) {

Yep, I should revise and send a v4 patch. Thanks.

>
>> +             if (di->nr_sectors == bd_sects)
>> +                     break;
>> +     }
>> +     if (di->nr_sectors == 0) {
>> +             fat_msg(sb, KERN_WARNING,
>> +                     "DOS volume lacks BPB and isn't a recognized floppy size (%ld sectors)",
>> +                     (long)bd_sects);
>
> sector_t can be u64 on 32-bit so one should really use %Lu and cast to
> u64.

Thanks, will fix.

Conrad

Re: [PATCH v2] fs: FAT: Add support for DOS 1.x formatted volumes

[Andrew Morton]

On Mon, 31 Mar 2014 15:21:17 -0700 Conrad Meyer <cse.cem@gmail.com> wrote:

> >> +     if (get_unaligned_le16(&b->sector_size) != 0 || b->sec_per_clus != 0 ||
> >> +             b->reserved != 0 || b->fats != 0 ||
> >> +             get_unaligned_le16(&b->dir_entries) != 0 ||
> >> +             get_unaligned_le16(&b->sectors) != 0 || b->media != 0 ||
> >> +             b->fat_length != 0 || b->secs_track != 0 || b->heads != 0 ||
> >> +             b->secs_track != 0 || b->heads != 0)
> >
> > Impressive!
> 
> I aim to please.

No great improvements immediately occur to me ;)

One could do

	/* nice comment */
	if (get_unaligned_le16(&b->sector_size) != 0)
		return;
	/* another nice comment */
	if (b->sec_per_clus != 0)
		return;
	...

but one would quickly run out of nice comments.

You could do s/ != 0//g.

> Not sure what would be better -- memcmp() part of the
> struct to a zeroed array?

memcmp would be hacky.  And possibly buggy if there are holes in the
struct, which is arch-dependent, shudder.

Re: [PATCH v2] fs: FAT: Add support for DOS 1.x formatted volumes

[Stephen Rothwell]

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On Mon, 31 Mar 2014 15:32:32 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Mon, 31 Mar 2014 15:21:17 -0700 Conrad Meyer <cse.cem@gmail.com> wrote:
> 
> > >> +     if (get_unaligned_le16(&b->sector_size) != 0 || b->sec_per_clus != 0 ||
> > >> +             b->reserved != 0 || b->fats != 0 ||
> > >> +             get_unaligned_le16(&b->dir_entries) != 0 ||
> > >> +             get_unaligned_le16(&b->sectors) != 0 || b->media != 0 ||
> > >> +             b->fat_length != 0 || b->secs_track != 0 || b->heads != 0 ||
> > >> +             b->secs_track != 0 || b->heads != 0)
> > >
> > > Impressive!
> > 
> > I aim to please.
> 
> No great improvements immediately occur to me ;)
> 
> One could do
> 
> 	/* nice comment */
> 	if (get_unaligned_le16(&b->sector_size) != 0)
> 		return;
> 	/* another nice comment */
> 	if (b->sec_per_clus != 0)
> 		return;
> 	...
> 
> but one would quickly run out of nice comments.
> 
> You could do s/ != 0//g.

You could also put the boolean expression in a (hopefully expressively
named) helper function and do the tests separately there.

-- 
Cheers,
Stephen Rothwell                    sfr@canb.auug.org.au

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