Re: Patch to fixe Data Acess error in dup_fd

[Vadim Lobanov <vlobanov@speakeasy.net>]

On Tue, 2006-11-14 at 23:42 +0300, Sergey Vlasov wrote:
> Yes, very interesting (although if the problem appeared only after 72
> hours of testing, it is hard to be sure that the bug is really fixed).

Yep. Could be random memory corruption from some other piece of the
code.

> > [...] The open_files value that
> > count_open_files() returns will always be a multiple of BITS_PER_LONG,
> > so no extraneous bits will ever be copied. It's a tad confusing since
> > count_open_files() does something a bit different than what its name
> > suggests.
> 
> Yes, then the logic looks fine.  (The comment in count_open_files()
> says "Find the last open fd", which is _almost_ what it does.)

It's always the details that getcha. :) There's other implicit tricks
that the code pulls, such as implicit requirements for minimum size
granularity of certain data structures. It basically comes down to
trying to read and understand all the surrounding code.

> There is also some unused code and slightly incorrect comment in
> dup_fd():
> 
> 	size = old_fdt->max_fdset;
> 
> ... here "size" is not used ....
> 
> 	/* if the old fdset gets grown now, we'll only copy up to "size" fds */
> 
> ... here "size" is not used either ....
> 
> 	size = (new_fdt->max_fds - open_files) * sizeof(struct file *);
> 
> The result of the first assignment to "size" is not used anywhere,
> even if it is mentioned in the comment.  However, the intent to keep
> the old size of fdset is noted again.

I read that comment in my mind like this: s/"size"/"open_files"/g. The
wording can definitely be improved, though. Any takers?

I also already sent a patch to Andrew a while ago to clean up that
unused assignment to "size", as part of a bigger fdtable-rework
patchset. This code is currently getting its testing in -mm for the time
being.

> > 0:mon> e
> > cpu 0x0: Vector: 300 (Data Access) at [c00000007ce2f7f0]
> >     pc: c000000000060d90: .dup_fd+0x240/0x39c
> >     lr: c000000000060d6c: .dup_fd+0x21c/0x39c
> >     sp: c00000007ce2fa70
> >    msr: 800000000000b032
> >    dar: ffffffff00000028
> >  dsisr: 40000000
> >   current = 0xc000000074950980
> >   paca    = 0xc000000000454500
> >     pid   = 27330, comm = bash
> > 
> > 0:mon> t
> > [c00000007ce2fa70] c000000000060d28 .dup_fd+0x1d8/0x39c (unreliable)
> > [c00000007ce2fb30] c000000000060f48 .copy_files+0x5c/0x88
> > [c00000007ce2fbd0] c000000000061f5c .copy_process+0x574/0x1520
> > [c00000007ce2fcd0] c000000000062f88 .do_fork+0x80/0x1c4
> > [c00000007ce2fdc0] c000000000011790 .sys_clone+0x5c/0x74
> > [c00000007ce2fe30] c000000000008950 .ppc_clone+0x8/0xc
> > 
> > The PC translates to:
> >         for (i = open_files; i != 0; i--) {
> >                 struct file *f = *old_fds++;
> >                 if (f) {
> >                         get_file(f);       <-- Data access error
> 
> So we probably got a bogus "struct file" pointer...
> 
> >                 } else {
> > 
> > And more info still:
> >         0:mon> r
> > R00 = ffffffff00000028   R16 = 00000000100e0000
> > R01 = c00000007ce2fa70   R17 = 000000000fff1d38
> > R02 = c00000000056cd20   R18 = 0000000000000000
> > R03 = c000000029f40a58   R19 = 0000000001200011
> > R04 = c000000029f442d8   R20 = c0000000a544a2a0
> > R05 = 0000000000000001   R21 = 0000000000000000
> > R06 = 0000000000000024   R22 = 0000000000000100
> > R07 = 0000001000000000   R23 = c00000008635f5e8
> > R08 = 0000000000000000   R24 = c0000000919c5448
> > R09 = 0000000000000024   R25 = 0000000000000100
> > R10 = 00000000000000dc   R26 = c000000086359c30
> > R11 = ffffffff00000000   R27 = c000000089e5e230
> > R12 = 0000000006bbd9e9   R28 = c00000000c8d3d80
> > R13 = c000000000454500   R29 = 0000000000000020
> > R14 = c00000007ce2fea0   R30 = c000000000491fc8
> > R15 = 00000000fcb2e770   R31 = c0000000b8369b08
> > pc  = c000000000060d90 .dup_fd+0x240/0x39c
> > lr  = c000000000060d6c .dup_fd+0x21c/0x39c
> > msr = 800000000000b032   cr  = 24242428
> > ctr = 0000000000000000   xer = 0000000000000000   trap =  300
> > dar = ffffffff00000028   dsisr = 40000000
> > -----------------------
> > 0:mon> di c000000000060d90 <==PC
> > c000000000060d90  7d200028      lwarx   r9,r0,r0
> > c000000000060d94  31290001      addic   r9,r9,1
> > c000000000060d98  7d20012d      stwcx.  r9,r0,r0
> > c000000000060d9c  40a2fff4      bne     c000000000060d90        # .dup_fd+0x240/0x39c
> 
>  From what little I know about PowerPC, this looks like an atomic
> increment of the memory word pointed to by r0, which contains
> 0xffffffff00000028 - definitely looks like a bogus address.  The
> offset of file->f_count should be 0x28 on a 64-bit architecture, so
> apparently we got f == 0xffffffff00000000 from *old_fds.  Something
> scribbled over that memory?

Looks very likely. It would be ironic if the fdsets somehow scribbled
there -- a series of all ones in the open_fds. :)

I sent a request asking which version of the kernel did this. 2.6.19-mm1
without hotfixes could crash like this, due to a bug in the new fdtable
code. Aside from that, and if noone can recognize some sort of magical
constant value in those registers above, it might be bisection time.

-- Vadim Lobanov


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