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SubjectFD array expansion problem
I've seen a problem with fd array expansion in the recent kernels (namely, in
2.3.18).

dup2 with a very large newfd argument causes an Oops in fd_install.

Unable to handle kernel paging request at virtual address c8052000
c012fae2
*pde = 07e0f063
Oops: 0002
CPU: 0
EIP: 0010:[<c012fae2>]
Using defaults from ksymoops -t elf32-i386 -a i386
EFLAGS: 00010286
eax: c8052000 ebx: 00000001 ecx: c384d8c0 edx: c61a8b60
esi: c384d8c0 edi: c61a8b60 ebp: 00004000 esp: c753ffb8
ds: 0018 es: 0018 ss: 0018
Process tt (pid: 23790, stackpage=c753f000)
Stack: 08048594 bffff8a8 c0107e80 00000000 00004000 00004000 00002000
08048594
bffff8a8 ffffffda 0000002b 0000002b 0000003f 400d4d71 00000023
00000206
bffff88c 0000002b
Call Trace: [<c0107e80>]
Code: 87 10 85 d2 74 16 8d 42 10 ff 4a 10 0f 94 c0 84 c0 74 09 52
>>EIP; c012fae2 <sys_dup2+d6/fc> <=====
Trace; c0107e80 <tracesys+18/23>
Code; c012fae2 <sys_dup2+d6/fc>
00000000 <_EIP>:
ode; c012fae2 <sys_dup2+d6/fc> <=====
0: 87 10 xchgl %edx,(%eax) <=====
Code; c012fae4 <sys_dup2+d8/fc>
2: 85 d2 testl %edx,%edx
Code; c012fae6 <sys_dup2+da/fc>
4: 74 16 je 1c <_EIP+0x1c> c012fafe <sys_dup2+f2/fc>
Code; c012fae8 <sys_dup2+dc/fc>
6: 8d 42 10 leal 0x10(%edx),%eax
Code; c012faeb <sys_dup2+df/fc>
9: ff 4a 10 decl 0x10(%edx)
Code; c012faee <sys_dup2+e2/fc>
c: 0f 94 c0 sete %al
Code; c012faf1 <sys_dup2+e5/fc>
f: 84 c0 testb %al,%al
Code; c012faf3 <sys_dup2+e7/fc>
11: 74 09 je 1c <_EIP+0x1c> c012fafe <sys_dup2+f2/fc>
Code; c012faf5 <sys_dup2+e9/fc>
13: 52 pushl %edx

BTW, does anyone know why fd array manipulations are performed via atomic
operation (xchg) in some places (fd_install, expand_fdset)?
It looks strange because
1) atomic operations don't cover all accesses;
2) these code parts are already protected by a write lock.

Best regards
Andrey V.
Savochkin

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