Re: Odd llseek semantics on directories

["Vladimir V. Saveliev" <vs@namesys.com>]

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Hello

Charles P. Wright wrote:
> Vladimir,
> 
> We actually came across this while working on Unionfs in the kernel,
> which uses lseek to inquire about directory positions and then resume
> directory reading operations (much like nfsd does).
> 

Ok, please try whether the attached patch makes reiser4 to behave similar to others.


> I've attached a user space program that demonstrates the behavior.
> 
> The relevant strace entries are:
> open("/mnt/r4/", O_RDONLY)              = 3
> getdents(3, /* 5 entries */, 268)       = 80
> lseek(3, 0, SEEK_CUR)                   = -1 ENOENT
> lseek(3, 4294967295, SEEK_SET)          = -1 EINVAL
> 
> The third line is the one of concern.
> 
> The same program run on another file system yields a trace like:
> open("/mnt", O_RDONLY)                  = 3
> getdents(3, /* 4 entries */, 268)       = 68
> lseek(3, 0, SEEK_CUR)                   = 2147483647
> lseek(3, 2147483647, SEEK_SET)          = 2147483647
> 
> Charles
> 
> On Mon, 2005-08-22 at 20:19 +0400, Vladimir V. Saveliev wrote:
>>Hello
>>
>>Charles P. Wright wrote:
>>>Hello,
>>>
>>>I've noticed that Reiser4's behavior deviates from other file systems
>>>when seeking with directories.  After reading a directory, if you run
>>>vfs_lseek(dir, 0, SEEK_CUR), then -ENOENT is returned.   This means that
>>>you can't pass the identifier back to vfs_lseek with SEEK_SET.
>>>
>>Would you please send your test program?
>>
>>------------------------------------------------------------------------
>>
>>#include <stdio.h>
>>#include <linux/types.h>
>>#include <linux/unistd.h>
>>#include <linux/dirent.h>
>>#include <sys/types.h>
>>#include <fcntl.h>
>>#include <sys/errno.h>
>>#include <unistd.h>
>>#include <stdlib.h>
>>
>>_syscall3(int, getdents, uint, fd, struct dirent *, dirp, uint, count);
>>
>>int main(int argc, char *argv[])
>>{
>>	struct dirent dirent;
>>	int fd;
>>	int nread;
>>	struct dirent *p;
>>	off_t offset;
>>
>>	if (argc != 2) {
>>		fprintf(stderr, "No dir specified\n");
>>		exit(1);
>>	}
>>
>>	if ((fd = open(argv[1], O_RDONLY)) < 0) {
>>		fprintf(stderr, "Could not open directory %s\n", argv[1]);
>>		exit(2);
>>	}
>>
>>	while ((nread = getdents(fd, &dirent, sizeof(dirent))) > 0) {
>>		p = &dirent;
>>		while (nread > 0) {
>>			printf("d_reclen = %d, d_name = %s, d_off = %lu\n", p->d_reclen, p->d_name, p->d_off);
>>			nread -= p->d_reclen;
>>			p = (struct dirent *)((char *)p + p->d_reclen);
>>		}
>>		offset = lseek(fd, 0, SEEK_CUR);
>>		printf("Offset is %llu\n", offset);

offset is off_t. off_t is long int in this program.
You should have therefore
printf("Offset is %ld\n", offset);

>>		if (lseek(fd, offset, SEEK_SET) < 0) {
>>			perror("lseek");
>>			exit(1);
>>		}
>>	}
>>	exit(0);
>>}


[-- Attachment #2: reiser4-fix-llseek.patch --]
[-- Type: text/plain, Size: 1442 bytes --]


This patch changes lseek part the end of reiser4 directory to not return ENOENT.


 fs/reiser4/plugin/dir/dir.c |    9 ++++++---
 1 files changed, 6 insertions(+), 3 deletions(-)

diff -puN fs/reiser4/plugin/dir/dir.c~reiser4-fix-llseek fs/reiser4/plugin/dir/dir.c
--- linux-2.6.12-rc6-mm1/fs/reiser4/plugin/dir/dir.c~reiser4-fix-llseek	2005-08-23 15:22:43.363256896 +0400
+++ linux-2.6.12-rc6-mm1-vs/fs/reiser4/plugin/dir/dir.c	2005-08-23 15:59:29.523105778 +0400
@@ -854,7 +854,10 @@ dir_rewind(struct file *dir, readdir_pos
 		memset(pos, 0, sizeof *pos);
 		return dir_go_to(dir, pos, tap);
 	} else if (destination >= inode->i_size)
-		return RETERR(-ENOENT);
+		/* seek past the end of directory */
+		dir->f_pos = inode->i_size;
+		return 0;
+	}
 
 	if (shift < 0) {
 		/* I am afraid of negative numbers */
@@ -1629,7 +1632,7 @@ readdir_common(struct file *f /* directo
 
  repeat:
 	result = dir_readdir_init(f, &tap, &pos);
-	if (result == 0) {
+	if (result == 0 && f->f_pos != inode->i_size) {
 		result = tap_load(&tap);
 		/* scan entries one by one feeding them to @filld */
 		while (result == 0) {
@@ -1669,7 +1672,7 @@ readdir_common(struct file *f /* directo
 
 		if (result >= 0)
 			f->f_version = inode->i_version;
-	} else if (result == -E_NO_NEIGHBOR || result == -ENOENT)
+	} else if (result == -E_NO_NEIGHBOR || f->f_pos == inode->i_size || result == -ENOENT)
 		result = 0;
 	tap_done(&tap);
 	detach_fsdata(f);

_