Function tracing is currently broken for all 32 bit MIPS platforms. When tracing is enabled, the kernel immediately hangs on boot. This is a result of commit b732d439cb43336cd6d7e804ecb2c81193ef63b0 that changes the kernel/trace/Kconfig file so that is no longer forces FRAME_POINTER when FUNCTION_TRACING is enabled. MIPS frame pointers are generally considered to be useless because they cannot be used to unwind the stack. Unfortunately the MIPS function tracing code has bugs that are masked by the use of frame pointers. This commit fixes the bugs so that MIPS frame pointers do not need to be enabled. The bugs are a result of the odd calling sequence used to call the trace routine. This calling sequence is inserted into every tracable function when the tracing CONFIG option is enabled. This sequence is generated for 32bit MIPS platforms by the compiler via the "-pg" flag. Part of the sequence is "addiu sp,sp,-8" in the delay slot after every call to the trace routine "_mcount" (some legacy thing where 2 arguments used to be pushed on the stack). The _mcount routine is expected to adjust the sp by +8 before returning. One of the bugs is that when tracing is disabled for a function, the "jalr _mcount" instruction is replaced with a nop, but the "addiu sp,sp,-8" is still executed and the stack pointer is left trashed. When frame pointers are enabled the problem is masked because any access to the stack is done through the frame pointer and the stack pointer is restored from the frame pointer when the function returns. This patch uses a branch likely instruction "bltzl zero, f1" instead of "nop" to disable the call because this instruction will not execute the "addiu sp,sp,-8" instruction in the delay slot. The other possible solution would be to nop out both the jalr and the "addiu sp,sp,-8", but this would need to be interrupt and SMP safe and would be much more intrusive. A few other bugs were fixed where the _mcount routine itself did not always fix the sp on return. Signed-off-by: Al Cooper <alcooperx@xxxxxxxxx> --- arch/mips/kernel/ftrace.c | 9 ++++++++- arch/mips/kernel/mcount.S | 14 ++++++++++---- 2 files changed, 18 insertions(+), 5 deletions(-) diff --git a/arch/mips/kernel/ftrace.c b/arch/mips/kernel/ftrace.c index 6a2d758..f761130 100644 --- a/arch/mips/kernel/ftrace.c +++ b/arch/mips/kernel/ftrace.c @@ -46,6 +46,13 @@ static inline int in_kernel_space(unsigned long ip) #define JUMP_RANGE_MASK ((1UL << 28) - 1) #define INSN_NOP 0x00000000 /* nop */ + +/* + * This branch likely instruction is used to nop the call to _mcount + * and skip the stack adjust instruction in the delay slot. + */ +#define INSN_NOP_SKIP 0x04020001 /* bltzl zero, f1 */ + #define INSN_JAL(addr) \ ((unsigned int)(JAL | (((addr) >> 2) & ADDR_MASK))) @@ -130,7 +137,7 @@ int ftrace_make_nop(struct module *mod, * If ip is in kernel space, no long call, otherwise, long call is * needed. */ - new = in_kernel_space(ip) ? INSN_NOP : INSN_B_1F; + new = in_kernel_space(ip) ? INSN_NOP_SKIP : INSN_B_1F; return ftrace_modify_code(ip, new); } diff --git a/arch/mips/kernel/mcount.S b/arch/mips/kernel/mcount.S index 4c968e7..78ac3cc 100644 --- a/arch/mips/kernel/mcount.S +++ b/arch/mips/kernel/mcount.S @@ -68,10 +68,10 @@ NESTED(ftrace_caller, PT_SIZE, ra) .globl _mcount _mcount: - b ftrace_stub + b ftrace_stub_restore_sp nop lw t1, function_trace_stop - bnez t1, ftrace_stub + bnez t1, ftrace_stub_restore_sp nop MCOUNT_SAVE_REGS @@ -96,13 +96,16 @@ ftrace_graph_call: .globl ftrace_stub ftrace_stub: RETURN_BACK +ftrace_stub_restore_sp: + PTR_ADDIU sp, 8 + RETURN_BACK END(ftrace_caller) #else /* ! CONFIG_DYNAMIC_FTRACE */ NESTED(_mcount, PT_SIZE, ra) lw t1, function_trace_stop - bnez t1, ftrace_stub + bnez t1, ftrace_stub_restore_sp nop PTR_LA t1, ftrace_stub PTR_L t2, ftrace_trace_function /* Prepare t2 for (1) */ @@ -118,7 +121,7 @@ NESTED(_mcount, PT_SIZE, ra) bne t1, t3, ftrace_graph_caller nop #endif - b ftrace_stub + b ftrace_stub_restore_sp nop static_trace: @@ -132,6 +135,9 @@ static_trace: .globl ftrace_stub ftrace_stub: RETURN_BACK +ftrace_stub_restore_sp: + PTR_ADDIU sp, 8 + RETURN_BACK END(_mcount) #endif /* ! CONFIG_DYNAMIC_FTRACE */ -- 1.7.6
