[PATCH v2 3/3] arm64: reliable stacktraces
From: Julien Thierry <hidden>
Date: 2018-08-20 08:28:27
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lkml
Hi Torsten, On 17/08/18 11:27, Torsten Duwe wrote:
This is more an RFC in the original sense: is this basically the correct approach? (as I had to tweak the save_stack API a bit). In particular the code does not detect interrupts and exception frames, and does not yet check whether the code address is valid. The latter check would also have to be omitted for the latest frame on other tasks' stacks. This would require some more tweaking. unwind_frame() now reports whether we had to stop normally or due to an error condition; walk_stackframe() will pass that info. __save_stack_trace() is used for a start to check the validity of a frame; maybe save_stack_trace_tsk_reliable() will need its own callback. The question is whether this change, once complete, is sufficient (as on powerpc) or whether a port of objtool is needed, like x86.
I'm not sure I understand the involvement of objtool in this. IIUC objtool just provides a way to check that the code manages stack frames consistently. It gives you more confidence in how the code manages the frame. If the semantics of HAVE_RELIABLE_STACKTRACE is "are you able to tell whether that stacktrace is good or corrupted?", the code looks mostly fine to me.
quoted hunk ↗ jump to hunk
I can dig into this myself and draw conclusions, but I'd prefer to have some input from ARM people here... Signed-off-by: Torsten Duwe <redacted>--- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig@@ -127,8 +127,9 @@ config ARM64 select HAVE_PERF_EVENTS select HAVE_PERF_REGS select HAVE_PERF_USER_STACK_DUMP - select HAVE_REGS_AND_STACK_ACCESS_API select HAVE_RCU_TABLE_FREE + select HAVE_REGS_AND_STACK_ACCESS_API + select HAVE_RELIABLE_STACKTRACE select HAVE_STACKPROTECTOR select HAVE_SYSCALL_TRACEPOINTS select HAVE_KPROBES --- a/arch/arm64/include/asm/stacktrace.h +++ b/arch/arm64/include/asm/stacktrace.h@@ -33,7 +33,7 @@ struct stackframe { }; extern int unwind_frame(struct task_struct *tsk, struct stackframe *frame); -extern void walk_stackframe(struct task_struct *tsk, struct stackframe *frame, +extern int walk_stackframe(struct task_struct *tsk, struct stackframe *frame, int (*fn)(struct stackframe *, void *), void *data); extern void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk);diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c index d5718a060672..fe0dd4745ff3 100644 --- a/arch/arm64/kernel/stacktrace.c +++ b/arch/arm64/kernel/stacktrace.c@@ -81,23 +81,27 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame) * both are NULL. */ if (!frame->fp && !frame->pc) - return -EINVAL; + return 1;
unwind_frame is not static so we should take care of other callers. I can see one in arch/arm64/kernel/time.c, profile_pc which checks "unwind_frame(...) < 0" to exit a loop, but now you have made 1 a valid stop condition. I think the other callers are just checking "unwind_frame(...) != 0" as stop condition so those should be fine. Perhaps adding a little comment on unwind_frame "0 -> Stack trace goes on, 1 -> reached end of stack trace, 2 -> stack trace looks" would be nice.
quoted hunk ↗ jump to hunk
return 0; } -void notrace walk_stackframe(struct task_struct *tsk, struct stackframe *frame, +int notrace walk_stackframe(struct task_struct *tsk, struct stackframe *frame, int (*fn)(struct stackframe *, void *), void *data) { while (1) { int ret; - if (fn(frame, data)) - break; + ret = fn(frame, data); + if (ret) + return ret; ret = unwind_frame(tsk, frame); if (ret < 0) + return ret; + if (ret > 0) break; } + return 0; } #ifdef CONFIG_STACKTRACE@@ -145,14 +149,15 @@ void save_stack_trace_regs(struct pt_regs *regs, struct stack_trace *trace) trace->entries[trace->nr_entries++] = ULONG_MAX; } -static noinline void __save_stack_trace(struct task_struct *tsk, +static noinline int __save_stack_trace(struct task_struct *tsk, struct stack_trace *trace, unsigned int nosched) { struct stack_trace_data data; struct stackframe frame; + int ret; if (!try_get_task_stack(tsk)) - return; + return -EBUSY; data.trace = trace; data.skip = trace->skip;@@ -171,11 +176,12 @@ static noinline void __save_stack_trace(struct task_struct *tsk, frame.graph = tsk->curr_ret_stack; #endif - walk_stackframe(tsk, &frame, save_trace, &data); + ret = walk_stackframe(tsk, &frame, save_trace, &data); if (trace->nr_entries < trace->max_entries) trace->entries[trace->nr_entries++] = ULONG_MAX; put_task_stack(tsk); + return ret; } EXPORT_SYMBOL_GPL(save_stack_trace_tsk);@@ -190,4 +196,12 @@ void save_stack_trace(struct stack_trace *trace) } EXPORT_SYMBOL_GPL(save_stack_trace); + +int save_stack_trace_tsk_reliable(struct task_struct *tsk, + struct stack_trace *trace) +{ + return __save_stack_trace(tsk, trace, 1); +} +EXPORT_SYMBOL_GPL(save_stack_trace_tsk_reliable); + #endif
-- Julien Thierry