From: Rabin Vincent on
Fix the mcount routines to build and run on a kernel built with the
Thumb-2 instruction set:

- Without the BSYM, the following assembler errors appear:

entry-common.S: Assembler messages:
entry-common.S:179: Error: invalid immediate for address calculation (value = 0x00000004)

- Without the orr, the lsb is not set on the pointer loaded from
ftrace_trace_function, but is set on BSYM(ftrace_stub), leading to the
comparison failing even when the pointer is pointing to ftrace_stub.

- The problem with the "mov lr, pc", is that it does not set the lsb when
storing the pc in lr. The called function returns with "bx lr", and the
mode changes to ARM. The blx is to avoid this.

Cc: Catalin Marinas <catalin.marinas(a)>
Signed-off-by: Rabin Vincent <rabin(a)>
arch/arm/kernel/entry-common.S | 8 +++++---
1 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index c3bdb05..573ed3b 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -167,7 +167,8 @@ ENTRY(__gnu_mcount_nc)
stmdb sp!, {r0-r3, lr}
ldr r0, =ftrace_trace_function
ldr r2, [r0]
- adr r0, ftrace_stub
+ THUMB( orr r2, r2, #1 )
+ adr r0, BSYM(ftrace_stub)
cmp r0, r2
bne gnu_trace
ldmia sp!, {r0-r3, ip, lr}
@@ -177,8 +178,9 @@ gnu_trace:
ldr r1, [sp, #20] @ lr of instrumented routine
mov r0, lr
sub r0, r0, #MCOUNT_INSN_SIZE
- mov lr, pc
- mov pc, r2
+ ARM( mov lr, pc )
+ ARM( mov pc, r2 )
+ THUMB( blx r2 )
ldmia sp!, {r0-r3, ip, lr}
mov pc, ip

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