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SubjectRe: kernel oops!

On Sun, 23 Jan 2005, ierdnah wrote:
> (gdb) disassemble pty_chars_in_buffer
> 0xc02c97a7 <pty_chars_in_buffer+23>: mov 0x28(%edx),%ecx **
> 0xc02c97aa <pty_chars_in_buffer+26>: test %ecx,%ecx
> 0xc02c97ac <pty_chars_in_buffer+28>: jne 0xc02c97b6
> 0xc02c97ae <pty_chars_in_buffer+30>: mov 0x4(%esp,1),%ebx
> 0xc02c97b2 <pty_chars_in_buffer+34>: add $0x8,%esp
> 0xc02c97b5 <pty_chars_in_buffer+37>: ret
> 0xc02c97b6 <pty_chars_in_buffer+38>: mov %edx,(%esp,1)
> 0xc02c97b9 <pty_chars_in_buffer+41>: call *0x28(%edx) **

Ahh, indeed. When I compiled this function, it kept 0x28(%edx) in a
register. Your config/compiler combination does not, so there actually is
a race condition if somebody changes the function pointer.

> this is another compiled kernel, but is compiled with the same .config
> file and same gcc version...because I only have the bzImage, how do I
> convert it to vmlinux?

Don't worry, it clearly shows that it's at least possible, and worth
looking at.

For testing, a patch like this might get rid of the problem by hiding the
race (but for all I know, gcc ends up re-loading it anyway) - you might
want to check the disassembly.

However, this patch is just for testing, to verify that your problem
really is that particular race. It's not a proper fix.

I suspect that pty's should always lock each others line disciplines too,
not just their "own" side.


--- 1.32/drivers/char/pty.c 2005-01-10 17:29:36 -08:00
+++ edited/drivers/char/pty.c 2005-01-23 09:49:16 -08:00
@@ -149,13 +149,15 @@
static int pty_chars_in_buffer(struct tty_struct *tty)
struct tty_struct *to = tty->link;
+ ssize_t (*chars_in_buffer)(struct tty_struct *);
int count;

- if (!to || !to->ldisc.chars_in_buffer)
+ /* We should get the line discipline lock for "tty->link" */
+ if (!to || !(chars_in_buffer = to->ldisc.chars_in_buffer))
return 0;

/* The ldisc must report 0 if no characters available to be read */
- count = to->ldisc.chars_in_buffer(to);
+ count = chars_in_buffer(to);

if (tty->driver->subtype == PTY_TYPE_SLAVE) return count;

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