[LLVMdev] how to track down a kernel miscompilation?

[Török Edwin]

Hi,

I am trying to build the Linux kernel with LLVM.
'ARCH=um' appears to work, now I am trying to get 'ARCH=x86' to work.

So far it seems there is something wrong with the boot vga code (it
finds no video modes), the acpi code, and the serial console code.

I am now trying to compile drivers/ with llvm-gcc and the rest with
gcc-4.2 (I have a wrapper script), I am compiling to native code, not
.bc files.

When compiling drivers/serial/serial_core.c with gcc-4.2 it boots fine,
but when compiled with llvm-gcc, I get a kernel panic.

How should I proceed further to find where the miscompilation is?
Can bugpoint help me here? (it takes ~10 seconds to compile and boot a
failing kernel with kvm/qemu)

This is 2.6.28-rc6-tip, and LLVM SVN r59914.

[    0.000000] Detected 2832.689 MHz processor.
[    0.004000] Console: colour VGA+ 80x25
[    0.004000] BUG: unable to handle kernel NULL pointer dereference at
00000000000000e8
[    0.004000] IP: [<ffffffff802b3680>] kmem_cache_alloc+0x30/0xa0
[    0.004000] PGD 0
[    0.004000] Thread overran stack, or stack corrupted
[    0.004000] Oops: 0000 [#1] PREEMPT SMP
[    0.004000] last sysfs file:
[    0.004000] CPU 0
[    0.004000] Modules linked in:
[    0.004000] Pid: 0, comm: swapper Not tainted 2.6.28-rc6-tip #165
[    0.004000] RIP: 0010:[<ffffffff802b3680>]  [<ffffffff802b3680>]
kmem_cache_alloc+0x30/0xa0
[    0.004000] RSP: 0018:ffffffff8075dd48  EFLAGS: 00010086
[    0.004000] RAX: 0000000000000000 RBX: ffffffff806e2320 RCX:
ffffffff80213546
[    0.004000] RDX: 0000000000002580 RSI: 00000000000000d0 RDI:
0000000000000000
[    0.004000] RBP: ffffffff8075dd68 R08: 0000000000000008 R09:
000000000000006e
[    0.004000] R10: 0000000000000000 R11: 0000000000000000 R12:
0000000000000286
[    0.004000] R13: 00000000000000d0 R14: ffffffff80847500 R15:
0000000000000000
[    0.004000] FS:  0000000000000000(0000) GS:ffffffff80757ec0(0000)
knlGS:0000000000000000
[    0.004000] CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
[    0.004000] CR2: 00000000000000e8 CR3: 0000000000201000 CR4:
00000000000006a0
[    0.004000] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
0000000000000000
[    0.004000] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7:
0000000000000400
[    0.004000] Process swapper (pid: 0, threadinfo ffffffff8075c000,
task ffffffff806e2320)
[    0.004000] Stack:
[    0.004000]  ffffffff806e2320 ffffffff806e2320 0000000000000000
ffffffff80847500
[    0.004000]  ffffffff8075dd88 ffffffff80213546 ffffffff806e2320
ffffffff8075c000
[    0.004000]  ffffffff8075dda8 ffffffff8020d6b9 0000000000000001
0000000000000000
[    0.004000] Call Trace:
[    0.004000]  [<ffffffff80213546>] init_fpu+0x106/0x130
[    0.004000]  [<ffffffff8020d6b9>] math_state_restore+0x89/0xc0
[    0.004000]  [<ffffffff80588739>] do_device_not_available+0x9/0x10
[    0.004000]  [<ffffffff8020d2a5>] device_not_available+0x15/0x20
[    0.004000]  [<ffffffff8045a5df>] ? uart_set_options+0xf/0xf0
[    0.004000]  [<ffffffff8045ca5f>] ? uart_parse_options+0x2f/0x90
[    0.004000]  [<ffffffff807896a8>] serial8250_console_setup+0xa8/0xc0
[    0.004000]  [<ffffffff8023801e>] register_console+0x28e/0x2f0
[    0.004000]  [<ffffffff807899c5>] serial8250_console_init+0x155/0x160
[    0.004000]  [<ffffffff80788542>] console_init+0x32/0x50
[    0.004000]  [<ffffffff80765d30>] start_kernel+0x230/0x3e0
[    0.004000]  [<ffffffff8076527c>] x86_64_start_reservations+0x7c/0xc0
[    0.004000]  [<ffffffff807653bb>] x86_64_start_kernel+0xcb/0xf0
[    0.004000] Code: 83 ec 20 4c 89 6c 24 10 48 89 1c 24 41 89 f5 4c 89
64 24 08 4c 89 74 24 18 48 8b 4d 08 9c 41 5c fa 65 8b 04 25 24 00 00 00
48 98 <48> 8b 94 c7 e8 00 00 00 48 8b 1a 44 8b 72 18 48 85 db 74 44 8b
[    0.004000] RIP  [<ffffffff802b3680>] kmem_cache_alloc+0x30/0xa0
[    0.004000]  RSP <ffffffff8075dd48>
[    0.004000] CR2: 00000000000000e8

Best regards,
--Edwin