[PATCH 6/6] vfs: introduce FMODE_NEG_OFFSET for allowing negative f_pos
OGAWA Hirofumi
> +static int
> +__negative_fpos_check(struct file *file, loff_t pos, size_t count)
> +{
> + /*
> + * pos or pos+count is negative here, check overflow.
> + * too big "count" will be caught in rw_verify_area().
> + */
> + if ((pos < 0) && (pos + count < pos))
> + return -EOVERFLOW;
> + if (file->f_mode & FMODE_NEG_OFFSET)
> + return 0;
> + return -EINVAL;
> +}
> +
> /*
> * rw_verify_area doesn't like huge counts. We limit
> * them to something that fits in "int" so that others
> @@ -222,8 +236,11 @@ int rw_verify_area(int read_write, struc
> if (unlikely((ssize_t) count < 0))
> return retval;
> pos = *ppos;
> - if (unlikely((pos < 0) || (loff_t) (pos + count) < 0))
> - return retval;
> + if (unlikely((pos < 0) || (loff_t) (pos + count) < 0)) {
> + retval = __negative_fpos_check(file, pos, count);
> + if (retval)
> + return retval;
> + }
>
> if (unlikely(inode->i_flock && mandatory_lock(inode))) {
> retval = locks_mandatory_area(
Um... How do lseek() work? It sounds like to violate error code range.
--
OGAWA Hirofumi <hiro...@mail.parknet.co.jp>
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KAMEZAWA Hiroyuki
This is for read-write. As far as I know,
- generic_file_llseek,
- default_llseek
- no_llseek
doesn't call this function.
Thanks,
-Kame
OGAWA Hirofumi
It seems to allow to set negative value to ->f_pos, right? So, lseek()
returns (uses) it?
Thanks.
--
OGAWA Hirofumi <hiro...@mail.parknet.co.jp>
KAMEZAWA Hiroyuki
yes. Some file (/dev/kmem) requires that.
> So, lseek() returns (uses) it?
lseek can return negative value, as far as I know.
Thanks,
-Kame
OGAWA Hirofumi
>> So, lseek() returns (uses) it?
>
> lseek can return negative value, as far as I know.
Umm..., how do you know the difference of -EOVERFLOW and fpos == -75?
Thanks.
--
OGAWA Hirofumi <hiro...@mail.parknet.co.jp>
OGAWA Hirofumi
>>> Um... How do lseek() work? It sounds like to violate error code range.
>>
>> This is for read-write. As far as I know,
>> - generic_file_llseek,
>> - default_llseek
>> - no_llseek
>>
>> doesn't call this function.
>
> It seems to allow to set negative value to ->f_pos, right? So, lseek()
> returns (uses) it?
BTW, another concern by negative "pos" value is, the following like code
pos >> shift_bits
it will break the above. So, I think it should be checked if not yet.
KAMEZAWA Hiroyuki
OGAWA Hirofumi <hiro...@mail.parknet.co.jp> wrote:
> OGAWA Hirofumi <hiro...@mail.parknet.co.jp> writes:
>
> >>> Um... How do lseek() work? It sounds like to violate error code range.
> >>
> >> This is for read-write. As far as I know,
> >> - generic_file_llseek,
> >> - default_llseek
> >> - no_llseek
> >>
> >> doesn't call this function.
> >
> > It seems to allow to set negative value to ->f_pos, right? So, lseek()
> > returns (uses) it?
>
> BTW, another concern by negative "pos" value is, the following like code
>
> pos >> shift_bits
>
> it will break the above. So, I think it should be checked if not yet.
Where do we check ?
FMODE_NEG_OFFSET is just used by /dev/mem and /proc/<pid>/mem. And I don't
think there are no additonal users. So, I myself don't have has such concerns...
Thanks,
-Kame
OGAWA Hirofumi
>> BTW, another concern by negative "pos" value is, the following like code
>>
>> pos >> shift_bits
>>
>> it will break the above. So, I think it should be checked if not yet.
>
> Where do we check ?
>
> FMODE_NEG_OFFSET is just used by /dev/mem and /proc/<pid>/mem. And I don't
> think there are no additonal users. So, I myself don't have has such concerns...
Sorry, it's just my concern. I'm not checking real path (e.g. vfs) of
related to /dev/mem, if there is no user of such code, it's ok.
Thanks.
--
OGAWA Hirofumi <hiro...@mail.parknet.co.jp>
KAMEZAWA Hiroyuki
OGAWA Hirofumi <hiro...@mail.parknet.co.jp> wrote:
> KAMEZAWA Hiroyuki <kamezaw...@jp.fujitsu.com> writes:
>
> >> So, lseek() returns (uses) it?
> >
> > lseek can return negative value, as far as I know.
>
> Umm..., how do you know the difference of -EOVERFLOW and fpos == -75?
>
Ah, sorry. I read wrong.
For /dev/mem, it uses its own lseek function which allows negative f_pos
value. Other usual file system doesn't allow negative f_pos.
It's ok not to return -EOVEFLOW for /dev/mem because there is no file end.
Thanks,
-Kame
OGAWA Hirofumi
> On Mon, 18 Jan 2010 10:38:27 +0900
> OGAWA Hirofumi <hiro...@mail.parknet.co.jp> wrote:
>
>> KAMEZAWA Hiroyuki <kamezaw...@jp.fujitsu.com> writes:
>>
>> >> So, lseek() returns (uses) it?
>> >
>> > lseek can return negative value, as far as I know.
>>
>> Umm..., how do you know the difference of -EOVERFLOW and fpos == -75?
>>
>
> Ah, sorry. I read wrong.
>
> For /dev/mem, it uses its own lseek function which allows negative f_pos
> value. Other usual file system doesn't allow negative f_pos.
>
> It's ok not to return -EOVEFLOW for /dev/mem because there is no file end.
No, no. I think it has the problem.
E.g. /dev/mem returns -75 as fpos, so, lseek(2) returns -75 to
userland. Then the userland (e.g. glibc) convert it as
error. I.e. finally, errno == -75, and lseek(3) returns -1, right?
Thanks.
--
OGAWA Hirofumi <hiro...@mail.parknet.co.jp>
KAMEZAWA Hiroyuki
OGAWA Hirofumi <hiro...@mail.parknet.co.jp> wrote:
> KAMEZAWA Hiroyuki <kamezaw...@jp.fujitsu.com> writes:
>
> > On Mon, 18 Jan 2010 10:38:27 +0900
> > OGAWA Hirofumi <hiro...@mail.parknet.co.jp> wrote:
> >
> >> KAMEZAWA Hiroyuki <kamezaw...@jp.fujitsu.com> writes:
> >>
> >> >> So, lseek() returns (uses) it?
> >> >
> >> > lseek can return negative value, as far as I know.
> >>
> >> Umm..., how do you know the difference of -EOVERFLOW and fpos == -75?
> >>
> >
> > Ah, sorry. I read wrong.
> >
> > For /dev/mem, it uses its own lseek function which allows negative f_pos
> > value. Other usual file system doesn't allow negative f_pos.
> >
> > It's ok not to return -EOVEFLOW for /dev/mem because there is no file end.
>
> No, no. I think it has the problem.
>
> E.g. /dev/mem returns -75 as fpos, so, lseek(2) returns -75 to
> userland. Then the userland (e.g. glibc) convert it as
> error. I.e. finally, errno == -75, and lseek(3) returns -1, right?
>
Maybe possible.
Hmm. Then, /dev/mem's llseek need some fix not to return pos < -PAGESIZE.
Wu-san, could you add additional bug fix to lseek()'s f_pos handling in
/dev/mem ?
Thanks,
-Kame
Wu, Fengguang
[replying from webmail, sorry for top-posting]
memory_lseek() calls force_successful_syscall_return() to force success on negative vals.
However that is a no-op for x86.
My experiment shows that lseek() does return negative pos. However,
manual says that "a value of (off_t) -1 is returned" on error. So it's OK
as long as your program is written as "err == -1" instead of "err < 0".
code:
err = lseek64(fd, addr, SEEK_SET);
if (err == -1)
perror("seek " FILENAME);
output:
# kmem-rw 0xffffffffa0094000
addr=0xffffffffa0094000 val=0x441f0fe5894855
strace:
open("/dev/kmem", O_RDWR) = 3
lseek(3, 18446744072099545088, SEEK_SET) = 18446744072099545088
read(3, "UH\211\345\17\37D\0"..., 8) = 8
Thanks,
Fengguang
________________________________________
From: KAMEZAWA Hiroyuki [kamezaw...@jp.fujitsu.com]
Sent: Monday, January 18, 2010 10:30 AM
To: OGAWA Hirofumi
Cc: Wu, Fengguang; Andrew Morton; Al Viro; Heiko Carstens; Christoph Hellwig; LKML; Eric Paris; Nick Piggin; Andi Kleen; David Howells; Jonathan Corbet; linux-...@vger.kernel.org
Subject: Re: [PATCH 6/6] vfs: introduce FMODE_NEG_OFFSET for allowing negative f_pos
KAMEZAWA Hiroyuki
"Wu, Fengguang" <fenggu...@intel.com> wrote:
> Hi,
>
> [replying from webmail, sorry for top-posting]
>
> memory_lseek() calls force_successful_syscall_return() to force success on negative vals.
> However that is a no-op for x86.
>
> My experiment shows that lseek() does return negative pos. However,
> manual says that "a value of (off_t) -1 is returned" on error. So it's OK
> as long as your program is written as "err == -1" instead of "err < 0".
>
On error, the kernel returns -EOVERFLOW (via %eax) and libc hides
it by
errno = EOVERFLOW
ret = -1
The problem discussed here is the kernel's return value. So, the kernel's
lseek should check that, I think.
Anyway, this lseek problem is not related to this patch itself and has
existed for very long time. Fixing it later by another patch is not very
bad, I think.
(I'm sorry I myself is not ready for writing a patch...)
Thaks,
KAMEZAWA Hiroyuki
"Wu, Fengguang" <fenggu...@intel.com> wrote:
>
> > On error, the kernel returns -EOVERFLOW (via %eax) and libc hides
> > it by
> > errno = EOVERFLOW
> > ret = -1
>
> Ah got it. How about the attached patch?
>
Seems good to me. Thank you very much.
Reviewed-by: KAMEZAWA Hiroyuki <kamezaw...@jp.fujitsu.com>