Messages in this thread | | | Date | Mon, 9 Nov 2020 18:16:10 -0500 | From | Steven Rostedt <> | Subject | Re: [PATCH 2/3 v4] ftrace/x86: Allow for arguments to be passed in to ftrace_regs by default |
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On Mon, 9 Nov 2020 12:10:43 +0100 Peter Zijlstra <peterz@infradead.org> wrote:
> > SYM_INNER_LABEL(ftrace_caller_op_ptr, SYM_L_GLOBAL) > > /* Load the ftrace_ops into the 3rd parameter */ > > movq function_trace_op(%rip), %rdx > > > > - /* regs go into 4th parameter (but make it NULL) */ > > - movq $0, %rcx > > + /* regs go into 4th parameter */ > > + leaq (%rsp), %rcx > > + > > + /* Only ops with REGS flag set should have CS register set */ > > + movq $0, CS(%rsp) > > > > SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL) > > call ftrace_stub > > You now seem to be relying on save_mcount_regs() resulting in a cleared > CS, however, AFAICT CS is uninitialized stack garbage.
We have two trampolines that are also used to copy for other trampolines that are dynamically created. There's this one (ftrace_caller) and then there's the regs one (ftrace_regs_caller). This one clears the CS in pt_regs to let the arch code know that the ftrace_regs is not a full pt_regs and anyone trying to get it with ftrace_get_regs() will get a NULL pointer. But the ftrace_regs_caller loads the CS register with __KERNEL_CS, which is non zero, and the ftrace_get_regs() will return the full pt_regs if it is set.
ftrace_caller: [..] movq $0, CS(%rsp) <- loads zero into pt_regs->cs for internal use only. [..] call callback
ftrace_regs_caller: [..] movq $__KERNEL_CS, %rcx movq %rcx, CS(%rsp) [..] call callback
Then in the callback we have:
callback(..., struct ftrace_regs *fregs) { struct pt_regs *regs = ftrace_get_regs(fregs); }
Where ftrace_get_regs is arch specific and returns for x86_64:
static __always_inline struct pt_regs * arch_ftrace_get_regs(struct ftrace_regs *fregs) { /* Only when FL_SAVE_REGS is set, cs will be non zero */ if (!fregs->regs.cs) return NULL; return &fregs->regs; }
What am I missing?
-- Steve
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