[PATCH 33/38] fs: fat: Support reading from a larger block size

[Heinrich Schuchardt]

Am 30. März 2023 23:32:22 MESZ schrieb Simon Glass <sjg at chromium.org>:
>At present it is not possible to read from some CDROM drives since the
>FAT sector size does not match the media's block size. Add a conversion
>option for this, so that reading is possible.
>
>This does increase SPL size for read-only FAT support by 25 bytes but
>all but 6 are covered by the previous patch. We could reduce the
>overhead of this feature to 0 bytes by making the code uglier (using
>a static variable).

512 and 2048 are not the only physical sector sizes. Some hard drives use 4096.

This change deserves a test case.

Best regards

Heinrich

>
>Signed-off-by: Simon Glass <sjg at chromium.org>
>---
>
> fs/fat/Kconfig     |  13 ++++++
> fs/fat/fat.c       | 107 ++++++++++++++++++++++++++++++++++++++++-----
> fs/fat/fat_write.c |   8 ++--
> 3 files changed, 114 insertions(+), 14 deletions(-)
>
>diff --git a/fs/fat/Kconfig b/fs/fat/Kconfig
>index 9bb11eac9f7a..b0aa888c6cc4 100644
>--- a/fs/fat/Kconfig
>+++ b/fs/fat/Kconfig
>@@ -22,3 +22,16 @@ config FS_FAT_MAX_CLUSTSIZE
> 	  is the smallest amount of disk space that can be used to hold a
> 	  file. Unless you have an extremely tight memory memory constraints,
> 	  leave the default.
>+
>+config FAT_BLK_XLATE
>+	bool "Enable FAT filesystem on a device with a larger block size"
>+	depends on FS_FAT
>+	help
>+	  This provides a simple translation mechanism for reading FAT
>+	  filesystems which don't use the same sector size as the underlying
>+	  media. For example, the FAT filesystem may use 512 bytes but the
>+	  media uses 2048, e.g. on a CDROM drive.
>+
>+	  This only supports the case where the FAT filesystem's sector size is
>+	  smaller than the media's block size. It does not support creating or
>+	  writing files.
>diff --git a/fs/fat/fat.c b/fs/fat/fat.c
>index f0df7988e172..02c6d55a99b3 100644
>--- a/fs/fat/fat.c
>+++ b/fs/fat/fat.c
>@@ -43,13 +43,93 @@ static struct disk_partition cur_part_info;
> #define DOS_FS_TYPE_OFFSET	0x36
> #define DOS_FS32_TYPE_OFFSET	0x52
> 
>-static int disk_read(__u32 block, __u32 nr_blocks, void *buf)
>+/**
>+ * disk_read_conv() - Read blocks and break them into smaller ones
>+ *
>+ * This is used when the FAT filesystem is hosted on a block device with a
>+ * block size greated than 512 bytes, e.g. the 2048 bytes of a CDROM drive. It
>+ * reads the blocks into a buffer and pulls out what is requested by the calling
>+ * function.
>+ *
>+ * It uses an internal 2KB buffer on the stack.
>+ *
>+ * @mydata: Filesystem information
>+ * @block: Block number to read, in terms of mydata->sect_size
>+ * @nr_blocks: Number of blocks to read, in terms of mydata->sect_size
>+ * @buf: Buffer for data
>+ */
>+static int disk_read_conv(fsdata *mydata, __u32 block, __u32 nr_blocks,
>+			  void *buf)
>+{
>+	uint factor, whole, remain, upto;
>+	ulong base, index;
>+	uint to_copy;
>+	u8 tbuf[2048];
>+	int ret;
>+
>+	log_debug("mydata %x, cur_dev %lx, block %x, nr_block %x\n",
>+		  mydata->sect_size, cur_dev->blksz, block, nr_blocks);
>+	if (mydata->sect_size > cur_dev->blksz ||
>+	    cur_dev->blksz > sizeof(tbuf)) {
>+		log_err("Block size %lx not supported\n", cur_dev->blksz);
>+		return -EIO;
>+	}
>+	factor = cur_dev->blksz / mydata->sect_size;
>+
>+	/* get the first partial block */
>+	base = cur_part_info.start + block / factor;
>+	index = block % factor;
>+	log_debug("cur_part_info.start %llx, block %x, base %lx, index %lx\n",
>+		  (unsigned long long)cur_part_info.start, block, base, index);
>+	ret = blk_dread(cur_dev, base, 1, tbuf);
>+	if (ret != 1)
>+		return -EIO;
>+
>+	to_copy = min((ulong)nr_blocks, factor - index);
>+	log_debug("to_copy %x\n", to_copy);
>+	memcpy(buf, tbuf + index * mydata->sect_size,
>+	       to_copy * mydata->sect_size);
>+	upto = to_copy;
>+
>+	/* load any whole blocks */
>+	remain = nr_blocks - upto;
>+	whole = remain / factor;
>+	log_debug("factor %x, whole %x, remain %x\n", factor, whole, remain);
>+	if (whole) {
>+		ret = blk_dread(cur_dev, base + 1, whole,
>+				buf + upto * mydata->sect_size);
>+		if (ret != whole)
>+			return -EIO;
>+		upto += whole * factor;
>+		remain = nr_blocks - upto;
>+	}
>+
>+	/* load any blocks at the end */
>+	log_debug("end: remain %x\n", remain);
>+	if (remain) {
>+		ret = blk_dread(cur_dev, base + 1 + whole, 1, tbuf);
>+		if (ret != 1)
>+			return -EIO;
>+		memcpy(buf + upto * mydata->sect_size, tbuf,
>+		       remain * mydata->sect_size);
>+		upto += remain;
>+	}
>+
>+	return upto;
>+}
>+
>+static int disk_read(fsdata *mydata, __u32 block, __u32 nr_blocks, void *buf)
> {
> 	ulong ret;
> 
> 	if (!cur_dev)
> 		return -1;
> 
>+	/* support converting from a larger block size */
>+	if (IS_ENABLED(CONFIG_FAT_BLK_XLATE) && mydata &&
>+	    mydata->sect_size != cur_dev->blksz)
>+		return disk_read_conv(mydata, block, nr_blocks, buf);
>+
> 	ret = blk_dread(cur_dev, cur_part_info.start + block, nr_blocks, buf);
> 
> 	if (ret != nr_blocks)
>@@ -66,7 +146,7 @@ int fat_set_blk_dev(struct blk_desc *dev_desc, struct disk_partition *info)
> 	cur_part_info = *info;
> 
> 	/* Make sure it has a valid FAT header */
>-	if (disk_read(0, 1, buffer) != 1) {
>+	if (disk_read(NULL, 0, 1, buffer) != 1) {
> 		cur_dev = NULL;
> 		return -1;
> 	}
>@@ -211,7 +291,7 @@ static __u32 get_fatent(fsdata *mydata, __u32 entry)
> 		if (flush_dirty_fat_buffer(mydata) < 0)
> 			return -1;
> 
>-		if (disk_read(startblock, getsize, bufptr) < 0) {
>+		if (disk_read(mydata, startblock, getsize, bufptr) < 0) {
> 			debug("Error reading FAT blocks\n");
> 			return ret;
> 		}
>@@ -265,7 +345,7 @@ get_cluster(fsdata *mydata, __u32 clustnum, __u8 *buffer, unsigned long size)
> 		debug("FAT: Misaligned buffer address (%p)\n", buffer);
> 
> 		while (size >= mydata->sect_size) {
>-			ret = disk_read(startsect++, 1, tmpbuf);
>+			ret = disk_read(mydata, startsect++, 1, tmpbuf);
> 			if (ret != 1) {
> 				debug("Error reading data (got %d)\n", ret);
> 				return -1;
>@@ -279,7 +359,7 @@ get_cluster(fsdata *mydata, __u32 clustnum, __u8 *buffer, unsigned long size)
> 		__u32 bytes_read;
> 		__u32 sect_count = size / mydata->sect_size;
> 
>-		ret = disk_read(startsect, sect_count, buffer);
>+		ret = disk_read(mydata, startsect, sect_count, buffer);
> 		if (ret != sect_count) {
> 			debug("Error reading data (got %d)\n", ret);
> 			return -1;
>@@ -292,7 +372,7 @@ get_cluster(fsdata *mydata, __u32 clustnum, __u8 *buffer, unsigned long size)
> 	if (size) {
> 		ALLOC_CACHE_ALIGN_BUFFER(__u8, tmpbuf, mydata->sect_size);
> 
>-		ret = disk_read(startsect, 1, tmpbuf);
>+		ret = disk_read(mydata, startsect, 1, tmpbuf);
> 		if (ret != 1) {
> 			debug("Error reading data (got %d)\n", ret);
> 			return -1;
>@@ -504,7 +584,7 @@ read_bootsectandvi(boot_sector *bs, volume_info *volinfo, int *fatsize)
> 		return -1;
> 	}
> 
>-	if (disk_read(0, 1, block) < 0) {
>+	if (disk_read(NULL, 0, 1, block) < 0) {
> 		debug("Error: reading block\n");
> 		goto fail;
> 	}
>@@ -586,9 +666,14 @@ static int get_fs_info(fsdata *mydata)
> 	mydata->sect_size = (bs.sector_size[1] << 8) + bs.sector_size[0];
> 	mydata->clust_size = bs.cluster_size;
> 	if (mydata->sect_size != cur_part_info.blksz) {
>-		printf("** FAT sector size mismatch (fs=%hu, dev=%lu)\n",
>-				mydata->sect_size, cur_part_info.blksz);
>-		return -1;
>+		if (IS_ENABLED(CONFIG_FAT_BLK_XLATE)) {
>+			printf("Warning: FAT sector size mismatch (fs=%u, dev=%lu): translating for read-only\n",
>+			       mydata->sect_size, cur_part_info.blksz);
>+		} else {
>+			printf("** FAT sector size mismatch (fs=%u, dev=%lu), see CONFIG_FAT_BLK_XLATE\n",
>+			       mydata->sect_size, cur_part_info.blksz);
>+			return -1;
>+		}
> 	}
> 	if (mydata->clust_size == 0) {
> 		printf("** FAT cluster size not set\n");
>@@ -846,7 +931,7 @@ void *fat_next_cluster(fat_itr *itr, unsigned int *nbytes)
> 	 * dent at a time and iteratively constructing the vfat long
> 	 * name.
> 	 */
>-	ret = disk_read(sect, read_size, itr->block);
>+	ret = disk_read(itr->fsdata, sect, read_size, itr->block);
> 	if (ret < 0) {
> 		debug("Error: reading block\n");
> 		return NULL;
>diff --git a/fs/fat/fat_write.c b/fs/fat/fat_write.c
>index 4d2d4db07fa6..baef52af6363 100644
>--- a/fs/fat/fat_write.c
>+++ b/fs/fat/fat_write.c
>@@ -474,7 +474,7 @@ static int set_fatent_value(fsdata *mydata, __u32 entry, __u32 entry_value)
> 
> 		startblock += mydata->fat_sect;
> 
>-		if (disk_read(startblock, getsize, bufptr) < 0) {
>+		if (disk_read(NULL, startblock, getsize, bufptr) < 0) {
> 			debug("Error reading FAT blocks\n");
> 			return -1;
> 		}
>@@ -709,7 +709,8 @@ get_set_cluster(fsdata *mydata, __u32 clustnum, loff_t pos, __u8 *buffer,
> 	/* partial write at beginning */
> 	if (pos) {
> 		wsize = min(bytesperclust - pos, size);
>-		ret = disk_read(startsect, mydata->clust_size, tmpbuf_cluster);
>+		ret = disk_read(NULL, startsect, mydata->clust_size,
>+				tmpbuf_cluster);
> 		if (ret != mydata->clust_size) {
> 			debug("Error reading data (got %d)\n", ret);
> 			return -1;
>@@ -775,7 +776,8 @@ get_set_cluster(fsdata *mydata, __u32 clustnum, loff_t pos, __u8 *buffer,
> 	/* partial write at end */
> 	if (size) {
> 		wsize = size;
>-		ret = disk_read(startsect, mydata->clust_size, tmpbuf_cluster);
>+		ret = disk_read(NULL, startsect, mydata->clust_size,
>+				tmpbuf_cluster);
> 		if (ret != mydata->clust_size) {
> 			debug("Error reading data (got %d)\n", ret);
> 			return -1;