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SubjectRe: Oops accessing file on 2048 bytes/sector vfat MO in 2.4.0
Moi!

On Fri, 26 Jan 2001, Stefan Meyknecht wrote:

> I receive a Kernel oops while copying a file from MO-drive (vfat) with
> 2048 bytes sector size. There is no problen with ext2 formatted MOs.
>
> I think it happens because the function pointer cfv_file_read of the
> struct cvf_format is initialized with null.

Yup, that's the cause.

> This oops is 100% reproducable with the kernels: 2.4.0, 2.4.1-pre3,
> 2.4.1-pre7 and 2.4.0-ac11 (probably all >= 2.4.0).

... and on a fair number < 2.4.0. The patch below will give you (dog slow)
read access to your FAT MOs. Apply in fs/fat/. And don't even think about
write() and mmap().

Regards,

Daniel.

--[snip]--
--- cvf.c.vanilla Mon Jan 1 22:46:20 2001
+++ cvf.c Mon Jan 1 23:31:23 2001
@@ -51,6 +51,9 @@
const char *buf,
size_t count,
loff_t *ppos);
+ssize_t bigblock_fat_file_read(struct file *filp, char *buf, size_t count,
+ loff_t *ppos);
+

struct cvf_format default_cvf = {
0, /* version - who cares? */
@@ -92,7 +95,7 @@
default_fat_access,
NULL,
default_fat_bmap,
- NULL,
+ bigblock_fat_file_read,
default_fat_file_write,
NULL,
NULL
--- file.c.vanilla Mon Jan 1 22:46:26 2001
+++ file.c Tue Jan 16 00:15:16 2001
@@ -4,6 +4,9 @@
* Written 1992,1993 by Werner Almesberger
*
* regular file handling primitives for fat-based filesystems
+ *
+ * 2001-1-1 Daniel Kobras <kobras@linux.de>:
+ * Added quick&dirty read operation for large sector media.
*/

#define ASC_LINUX_VERSION(V, P, S) (((V) * 65536) + ((P) * 256) + (S))
@@ -114,6 +117,56 @@
return retval;
}

+/* This is a hack. No readahead and other fancy stuff, but hopefully enough
+ * to get MOs working again. [dk]
+ * FIXME: Not sure whether I got error checking right.
+ */
+ssize_t bigblock_fat_file_read(struct file *filp, char *buf, size_t count,
+ loff_t *ppos)
+{
+ struct inode *inode = filp->f_dentry->d_inode;
+ int phys, pos;
+ struct buffer_head *bh;
+ size_t to_go, done;
+ char *buf_start = buf;
+
+ /* Taken from 2.2 source. */
+ if (!S_ISREG(inode->i_mode) && !S_ISLNK(inode->i_mode))
+ return -EINVAL;
+
+ if (*ppos > inode->i_size || !count)
+ return 0;
+
+ if (inode->i_size - *ppos < count)
+ count = inode->i_size - *ppos;
+
+ pos = *ppos>>SECTOR_BITS;
+ to_go = SECTOR_SIZE - (*ppos&(SECTOR_SIZE-1));
+ goto _start;
+
+ do {
+ to_go = SECTOR_SIZE;
+_start:
+ phys = fat_bmap(inode, pos++);
+ if (!phys)
+ return -EIO;
+ bh = fat_bread(inode->i_sb, phys);
+ if (!bh)
+ return -EIO;
+ done = to_go > count ? count : to_go;
+ if (copy_to_user(buf, bh->b_data, done)) {
+ fat_brelse(inode->i_sb, bh);
+ return -EFAULT;
+ }
+ fat_brelse(inode->i_sb, bh);
+ buf += done;
+ *ppos += done;
+ } while ((count -= done));
+
+ return buf - buf_start;
+}
+
+
void fat_truncate(struct inode *inode)
{
struct msdos_sb_info *sbi = MSDOS_SB(inode->i_sb);
--- inode.c.vanilla Tue Jan 2 00:36:18 2001
+++ inode.c Tue Jan 2 00:22:04 2001
@@ -820,6 +820,9 @@
inode->i_size = CF_LE_L(de->size);
inode->i_op = &fat_file_inode_operations;
inode->i_fop = &fat_file_operations;
+ /* FIXME: mmap is broken with large hwblocks! [dk] */
+ if (sb->s_blocksize > 512)
+ inode->i_fop->mmap = NULL;
inode->i_mapping->a_ops = &fat_aops;
MSDOS_I(inode)->mmu_private = inode->i_size;
}



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