Messages in this thread | | | From | "Wu, Fengguang" <> | Date | Mon, 18 Jan 2010 11:15:38 +0800 | Subject | RE: [PATCH 6/6] vfs: introduce FMODE_NEG_OFFSET for allowing negative f_pos |
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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".
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 [kamezawa.hiroyu@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-fsdevel@vger.kernel.org Subject: Re: [PATCH 6/6] vfs: introduce FMODE_NEG_OFFSET for allowing negative f_pos
On Mon, 18 Jan 2010 11:13:04 +0900 OGAWA Hirofumi <hirofumi@mail.parknet.co.jp> wrote:
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> writes: > > > On Mon, 18 Jan 2010 10:38:27 +0900 > > OGAWA Hirofumi <hirofumi@mail.parknet.co.jp> wrote: > > > >> KAMEZAWA Hiroyuki <kamezawa.hiroyu@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
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