path: root/arch/arm/kernel/entry-common.S
diff options
authorRussell King <rmk+kernel@arm.linux.org.uk>2012-10-15 00:16:49 +0100
committerLinus Torvalds <torvalds@linux-foundation.org>2012-10-15 07:57:34 -0700
commit68687c842caefd5386d47a571fb4725df3556891 (patch)
treed85ddbbffa59032585dbb02f3432fce31fd451f9 /arch/arm/kernel/entry-common.S
parentddffeb8c4d0331609ef2581d84de4d763607bd37 (diff)
ARM: fix oops on initial entry to userspace with Thumb2 kernels
Daniel Mack reports an oops at boot with the latest kernels: Internal error: Oops - undefined instruction: 0 [#1] SMP THUMB2 Modules linked in: CPU: 0 Not tainted (3.6.0-11057-g584df1d #145) PC is at cpsw_probe+0x45a/0x9ac LR is at trace_hardirqs_on_caller+0x8f/0xfc pc : [<c03493de>] lr : [<c005e81f>] psr: 60000113 sp : cf055fb0 ip : 00000000 fp : 00000000 r10: 00000000 r9 : 00000000 r8 : 00000000 r7 : 00000000 r6 : 00000000 r5 : c0344555 r4 : 00000000 r3 : cf057a40 r2 : 00000000 r1 : 00000001 r0 : 00000000 Flags: nZCv IRQs on FIQs on Mode SVC_32 ISA ARM Segment user Control: 50c5387d Table: 8f3f4019 DAC: 00000015 Process init (pid: 1, stack limit = 0xcf054240) Stack: (0xcf055fb0 to 0xcf056000) 5fa0: 00000001 00000000 00000000 00000000 5fc0: cf055fb0 c000d1a8 00000000 00000000 00000000 00000000 00000000 00000000 5fe0: 00000000 be9b3f10 00000000 b6f6add0 00000010 00000000 aaaabfaf a8babbaa The analysis of this is as follows. In init/main.c, we issue: kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND); This creates a new thread, which falls through to the ret_from_fork assembly, with r4 set NULL and r5 set to kernel_init. You can see this in your oops dump register set - r5 is 0xc0344555, which is the address of kernel_init plus 1 which marks the function as Thumb code. Now, let's look at this code a little closer - this is what the disassembly looks like: c000d180 <ret_from_fork>: c000d180: f03a fe08 bl c0047d94 <schedule_tail> c000d184: 2d00 cmp r5, #0 c000d186: bf1e ittt ne c000d188: 4620 movne r0, r4 c000d18a: 46fe movne lr, pc <-- XXXXXXX c000d18c: 46af movne pc, r5 c000d18e: 46e9 mov r9, sp c000d190: ea4f 3959 mov.w r9, r9, lsr #13 c000d194: ea4f 3949 mov.w r9, r9, lsl #13 c000d198: e7c8 b.n c000d12c <ret_to_user> c000d19a: bf00 nop c000d19c: f3af 8000 nop.w This code was introduced in 9fff2fa0db911 (arm: switch to saner kernel_execve() semantics). I have marked one instruction, and it's the significant one - I'll come back to that later. Eventually, having had a successful call to kernel_execve(), kernel_init() returns zero. In returning, it uses the value in 'lr' which was set by the instruction I marked above. Unfortunately, this causes lr to contain 0xc000d18e - an even address. This switches the ISA to ARM on return but with a non word aligned PC value. So, what do we end up executing? Well, not the instructions above - yes the opcodes, but they don't mean the same thing in ARM mode. In ARM mode, it looks like this instead: c000d18c: 46e946af strbtmi r4, [r9], pc, lsr #13 c000d190: 3959ea4f ldmdbcc r9, {r0, r1, r2, r3, r6, r9, fp, sp, lr, pc}^ c000d194: 3949ea4f stmdbcc r9, {r0, r1, r2, r3, r6, r9, fp, sp, lr, pc}^ c000d198: bf00e7c8 svclt 0x0000e7c8 c000d19c: 8000f3af andhi pc, r0, pc, lsr #7 c000d1a0: e88db092 stm sp, {r1, r4, r7, ip, sp, pc} c000d1a4: 46e81fff ; <UNDEFINED> instruction: 0x46e81fff c000d1a8: 8a00f3ef bhi 0xc004a16c c000d1ac: 0a0cf08a beq 0xc03493dc I have included more above, because it's relevant. The PSR flags which we can see in the oops dump are nZCv, so Z and C are set. All the above ARM instructions are not executed, except for two. c000d1a0, which has no writeback, and writes below the current stack pointer (and that data is lost when we take the next exception.) The other instruction which is executed is c000d1ac, which takes us to... 0xc03493dc. However, remember that bit 1 of the PC got set. So that makes the PC value 0xc03493de. And that value is the value we find in the oops dump for PC. What is the instruction here when interpreted in ARM mode? 0: f71e150c ; <UNDEFINED> instruction: 0xf71e150c and there we have our undefined instruction (remember that the 'never' condition code, 0xf, has been deprecated and is now always executed as it is now being used for additional instructions.) This path also nicely explains the state of the stack we see in the oops dump too. The above is a consistent and sane story for how we got to the oops dump, which all stems from the instruction at 0xc000d18a being wrong. Reported-by: Daniel Mack <zonque@gmail.com> Tested-by: Daniel Mack <zonque@gmail.com> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'arch/arm/kernel/entry-common.S')
1 files changed, 2 insertions, 2 deletions
diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index 417bac1846bd..34711757ba59 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -88,9 +88,9 @@ ENTRY(ret_from_fork)
bl schedule_tail
cmp r5, #0
movne r0, r4
- movne lr, pc
+ adrne lr, BSYM(1f)
movne pc, r5
- get_thread_info tsk
+1: get_thread_info tsk
b ret_slow_syscall