seq_file:update file->f_pos when lseek() to m->read_pos
PATCH 1/1 seq_file:update file->f_pos when lseek() to m->read_pos
After pread(), file->f_pos and m->read_pos get different, and lseek() to m->read_pos did not update file->f_pos, then a subsequent read may read from a wrong position, the following program shows the problem:
char str1[32] = { 0 };
char str2[32] = { 0 };
int poffset = 10;
int count = 20;
/*open any seq file*/
int fd = open("/proc/modules", O_RDONLY);
pread(fd, str1, count, poffset);
printf("pread:%s\n", str1);
/*seek to where m->read_pos is*/
lseek(fd, poffset+count, SEEK_SET);
/*supposed to read from poffset+count, but this read from position 0*/
read(fd, str2, count);
printf("read:%s\n", str2);--- fs/seq_file.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
fs/seq_file.c +2 -1
@@ -328,7 +328,8 @@ loff_t seq_lseek(struct file *file, loff_t offset, int whence) m->read_pos = offset; retval = file->f_pos = offset; } - } + }else + file->f_pos = offset; } file->f_version = m->version;mutex_unlock (&m->lock);
Signature · 1 line
1.8.1.2
Quoted 34 lines · Jiaxing Wang
After
pread(),file->f_posandm->read_posget different, andlseek()tom->read_posdid not updatefile->f_pos, then a subsequent read may read from a wrong position, the following program shows the problem:char str1[32] = { 0 }; char str2[32] = { 0 }; int poffset = 10; int count = 20; /*open any seq file*/ int fd = open("/proc/modules", O_RDONLY); pread(fd, str1, count, poffset); printf("pread:%s\n", str1); /*seek to where m->read_pos is*/ lseek(fd, poffset+count, SEEK_SET); /*supposed to read from poffset+count, but this read from position 0*/ read(fd, str2, count); printf("read:%s\n", str2);Signed-off-by Jiaxing Wang <[email protected]>--- fs/seq_file.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)fs/seq_file.c +0
-0
fs/seq_file.c loff_t seq_lseek(struct file *file, loff_t offset, int whence) · line 335
Quoted 9 lines
fs/seq_file.c +2
-1@@ -328,7 +328,8 @@ loff_t seq_lseek(struct file *file, loff_t offset, int whence) m->read_pos = offset; retval = file->f_pos = offset; } - } + }else + file->f_pos = offset; } file->f_version = m->version;mutex_unlock (&m->lock);
This does not appear to be a problem, in linux man page, the behaver seems clearly defined:
DESCRIPTION
pread() reads up to count bytes from file descriptor fd at
offset off-set (from the start of the file) into the buffer starting at buf. The file offset is not changed.
pwrite() writes up to count bytes from the buffer starting at
buf to the file descriptor fd at offset offset. The file offset is not changed.
Quoted 46 lines · fangdong
Quoted 34 lines · Jiaxing Wang
After
pread(),file->f_posandm->read_posget different, andlseek()tom->read_posdid not updatefile->f_pos, then a subsequent read may read from a wrong position, the following program shows the problem:char str1[32] = { 0 }; char str2[32] = { 0 }; int poffset = 10; int count = 20; /*open any seq file*/ int fd = open("/proc/modules", O_RDONLY); pread(fd, str1, count, poffset); printf("pread:%s\n", str1); /*seek to where m->read_pos is*/ lseek(fd, poffset+count, SEEK_SET); /*supposed to read from poffset+count, but this read from position 0*/ read(fd, str2, count); printf("read:%s\n", str2);Signed-off-by Jiaxing Wang <[email protected]>--- fs/seq_file.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)fs/seq_file.c +0
-0fs/seq_file.c loff_t seq_lseek(struct file *file, loff_t offset, int whence) · line 335
Quoted 9 lines
fs/seq_file.c +2
-1@@ -328,7 +328,8 @@ loff_t seq_lseek(struct file *file, loff_t offset, int whence) m->read_pos = offset; retval = file->f_pos = offset; } - } + }else + file->f_pos = offset; } file->f_version = m->version;mutex_unlock (&m->lock);This does not appear to be a problem, in linux man page, the behaver seems clearly defined:
Quoted 10 lines
DESCRIPTION
pread()reads up to count bytes from file descriptor fd at offset off-set (from the start of the file) into the buffer starting at buf. The file offset is not changed.
pwrite()writes up to count bytes from the buffer starting at buf to the file descriptor fd at offset offset. The file offset is not changed.Signature · 4 lines
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There's no problem that pread() don't change file->f_pos, but I think lseek() should have changed it. Any comments from other people, and Alexander?
Hi Jiaxing,
Please refer to inline comment.:)
Quoted 50 lines · Jiaxing Wang
Quoted 49 lines · fangdong
Quoted 34 lines · Jiaxing Wang
After
pread(),file->f_posandm->read_posget different, andlseek()tom->read_posdid not updatefile->f_pos, then a subsequent read may read from a wrong position, the following program shows the problem:char str1[32] = { 0 }; char str2[32] = { 0 }; int poffset = 10; int count = 20; /*open any seq file*/ int fd = open("/proc/modules", O_RDONLY); pread(fd, str1, count, poffset); printf("pread:%s\n", str1); /*seek to where m->read_pos is*/ lseek(fd, poffset+count, SEEK_SET); /*supposed to read from poffset+count, but this read from position 0*/ read(fd, str2, count); printf("read:%s\n", str2);Signed-off-by Jiaxing Wang <[email protected]>--- fs/seq_file.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)fs/seq_file.c +0
-0fs/seq_file.c loff_t seq_lseek(struct file *file, loff_t offset, int whence) · line 335
Quoted 9 lines
fs/seq_file.c +2
-1@@ -328,7 +328,8 @@ loff_t seq_lseek(struct file *file, loff_t offset, int whence) m->read_pos = offset; retval = file->f_pos = offset; } - } + }else + file->f_pos = offset; } file->f_version = m->version;mutex_unlock (&m->lock);This does not appear to be a problem, in linux man page, the behaver seems clearly defined:
DESCRIPTION
pread()reads up to count bytes from file descriptor fd at offset off-set (from the start of the file) into the buffer starting at buf. The file offset is not changed.
Quoted 8 lines
Quoted 7 lines
pwrite()writes up to count bytes from the buffer starting at buf to the file descriptor fd at offset offset. The file offset is not changed.Signature · 4 lines
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/There's no problem that
pread()don't changefile->f_pos, but I thinklseek()should have changed it.
No, the way that seq_seek deal with pos is right. Remember this seq file, it is an iterator interface, e.g. it provides infos of each element of a list, so the we should make sure that the info we read from seq file is the whole infos of each entry, so seq_lseek can not set pos to a optional point, only can on the points that divided by a whole infos size of each entry. If we can not set pos on the special position, ste it as 0, I think it's reasonable.
Thanks,
Gu
Quoted 7 lines
Any comments from other people, and Alexander?
--
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More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Quoted 34 lines · Jiaxing Wang
After
pread(),file->f_posandm->read_posget different, andlseek()tom->read_posdid not updatefile->f_pos, then a subsequent read may read from a wrong position, the following program shows the problem:char str1[32] = { 0 }; char str2[32] = { 0 }; int poffset = 10; int count = 20; /*open any seq file*/ int fd = open("/proc/modules", O_RDONLY); pread(fd, str1, count, poffset); printf("pread:%s\n", str1); /*seek to where m->read_pos is*/ lseek(fd, poffset+count, SEEK_SET); /*supposed to read from poffset+count, but this read from position 0*/ read(fd, str2, count); printf("read:%s\n", str2);Signed-off-by Jiaxing Wang <[email protected]>--- fs/seq_file.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)fs/seq_file.c +0
-0
fs/seq_file.c loff_t seq_lseek(struct file *file, loff_t offset, int whence) · line 335
Quoted 9 lines
fs/seq_file.c +2
-1@@ -328,7 +328,8 @@ loff_t seq_lseek(struct file *file, loff_t offset, int whence) m->read_pos = offset; retval = file->f_pos = offset; } - } + }else + file->f_pos = offset; } file->f_version = m->version;mutex_unlock (&m->lock);
This does not appear to be a problem, in linux man page, the behaver seems clearly defined:
DESCRIPTION
pread() reads up to count bytes from file descriptor fd at
offset off-set (from the start of the file) into the buffer starting at buf. The file offset is not changed.
pwrite() writes up to count bytes from the buffer starting at
buf to the file descriptor fd at offset offset. The file
offset is not changed.