[LLVMdev] how to track down a kernel miscompilation?

[Andrew Lenharth]

To get x86 to work, you need to compile /arch/x86/boot and
/arch/x86/boot/compressed with gcc.  The rest can be compiled with
llvm-gcc.  The only adjustment you need to make is to
/arch/x86/kernel/signal_32.c to fix the stack offset in sys_sigreturn.

Also you must compile with -O0.  instcombine is causing the crashes
you are seeing.  I know mem2reg, sccp, simplifycfg, and dce result in
a working kernel.  (My build script does llvm-gcc -> opt with custom
passes for my project -> llc.  You also must compile arch/x86/lib with
-O2 as it has inline asm that the fast regalloc can't handle.

Last time I tried to specifically find the transform pass that caused
a crash, I had to find the specific file that was affected by the
transform and compare the pre and post transformed files to see what
was wrong.  At the time (~ 1 year ago) it was almost always
instcombine and almost always volatile related.  Now bugpoint has some
ways to run the code by some external agent which I haven't
investigated yet.

Oh and if you are trying to construct an entire kernel bitcode file,
the following rule will help:

cmd_ll_as       = echo "module asm \"" > $@ ; $(ASMCC) $(a_flags) -c
-o - -S $< |perl -npe s/\"/\\\\22/g >> $@ ; echo "\"" >> $@

Andrew

On Mon, Nov 24, 2008 at 5:49 AM, Török Edwin <edwintorok at gmail.com> wrote:
> 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
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