On 10/26/21 7:05 AM, Mark Rutland wrote: > On Mon, Oct 25, 2021 at 05:49:25PM +0100, Mark Rutland wrote: >> From f3e66ca75aff3474355839f72d123276028204e1 Mon Sep 17 00:00:00 2001 >> From: Mark Rutland <mark.rutland@xxxxxxx> >> Date: Mon, 25 Oct 2021 13:23:11 +0100 >> Subject: [PATCH] arm64: ftrace: use HAVE_FUNCTION_GRAPH_RET_ADDR_PTR >> >> When CONFIG_FUNCTION_GRAPH_TRACER is selected, and the function graph: >> tracer is in use, unwind_frame() may erroneously asscociate a traced >> function with an incorrect return address. This can happen when starting >> an unwind from a pt_regs, or when unwinding across an exception >> boundary. >> >> The underlying problem is that ftrace_graph_get_ret_stack() takes an >> index offset from the most recent entry added to the fgraph return >> stack. We start an unwind at offset 0, and increment the offset each >> time we encounter `return_to_handler`, which indicates a rewritten >> return address. This is broken in two cases: >> >> * Between creating a pt_regs and starting the unwind, function calls may >> place entries on the stack, leaving an abitrary offset which we can >> only determine by performing a full unwind from the caller of the >> unwind code. While this initial unwind is open-coded in >> dump_backtrace(), this is not performed for other unwinders such as >> perf_callchain_kernel(). >> >> * When unwinding across an exception boundary (whether continuing an >> unwind or starting a new unwind from regs), we always consume the LR >> of the interrupted context, though this may not have been live at the >> time of the exception. Where the LR was not live but happened to >> contain `return_to_handler`, we'll recover an address from the graph >> return stack and increment the current offset, leaving subsequent >> entries off-by-one. >> >> Where the LR was not live and did not contain `return_to_handler`, we >> will still report an erroneous address, but subsequent entries will be >> unaffected. > > It turns out I had this backwards, and we currently always *skip* the LR > when unwinding across regs, because: > > * The entry assembly creates a synthetic frame record with the original > FP and the ELR_EL1 value (i.e. the PC at the point of the exception), > skipping the LR. > > * In arch_stack_walk() we start the walk from regs->pc, and continue > with the frame record, skipping the LR. > > * In the existing dump_backtrace, we skip until we hit a frame record > whose FP value matches the FP in the regs (i.e. the synthetic frame > record created by the entry assembly). That'll dump the ELR_EL1 value, > then continue to the next frame record, skipping the LR. > > So case two is bogus, and only case one can happen today. This cleanup > shouldn't trigger the WARN_ON_ONCE() in unwind_frame(), and we can fix > the missing LR entry in a subsequent cleanup. > OK. Thanks. Madhavan
