Re: [PATCH v5 11/34] function_graph: Have the instances use their own ftrace_ops for filtering

[Mark Rutland]

On Mon, Jan 08, 2024 at 02:21:03PM +0000, Mark Rutland wrote:
> On Mon, Jan 08, 2024 at 12:25:55PM +0000, Mark Rutland wrote:
> > We also have HAVE_FUNCTION_GRAPH_RET_ADDR_PTR, but since the return address is
> > not on the stack at the point function-entry is intercepted we use the FP as
> > the retp value -- in the absence of tail calls this will be different between a
> > caller and callee.
> 
> Ah; I just spotted that this patch changed that in ftrace_graph_func(), which
> is the source of the bug. 
> 
> As of this patch, we use the address of fregs->lr as the retp value, but the
> unwinder still uses the FP value, and so when unwind_recover_return_address()
> calls ftrace_graph_ret_addr(), the retp value won't match the expected entry on
> the fgraph ret_stack, resulting in failing to find the expected entry.
> 
> Since the ftrace_regs only exist transiently during function entry/exit, it's
> possible for a stackframe to reuse that same address on the stack, which would
> result in finding a different entry by mistake.
> 
> The diff below restores the existing behaviour and fixes the issue for me.
> Could you please fold that into this patch?
> 
> On a separate note, looking at how this patch changed arm64's
> ftrace_graph_func(), do we need similar changes to arm64's
> prepare_ftrace_return() for the old-style mcount based ftrace?
> 
> Mark.
> 
> ---->8----
> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> index 205937e04ece..329092ce06ba 100644
> --- a/arch/arm64/kernel/ftrace.c
> +++ b/arch/arm64/kernel/ftrace.c
> @@ -495,7 +495,7 @@ void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
>         if (bit < 0)
>                 return;
>  
> -       if (!function_graph_enter_ops(*parent, ip, fregs->fp, parent, gops))
> +       if (!function_graph_enter_ops(*parent, ip, fregs->fp, (void *)fregs->fp, gops))
>                 *parent = (unsigned long)&return_to_handler;
>  
>         ftrace_test_recursion_unlock(bit);

Thinking some more, this line gets excessively long when we pass the fregs too,
so it's probably worth adding a local variable for fp, i.e. the diff below.

Mark.

---->8----
diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index 205937e04ece..d4e142ef4686 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -481,8 +481,9 @@ void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
 void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
                       struct ftrace_ops *op, struct ftrace_regs *fregs)
 {
-       unsigned long *parent = &fregs->lr;
        struct fgraph_ops *gops = container_of(op, struct fgraph_ops, ops);
+       unsigned long *parent = &fregs->lr;
+       unsigned long fp = fregs->fp;
        int bit;
 
        if (unlikely(ftrace_graph_is_dead()))
@@ -495,7 +496,7 @@ void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
        if (bit < 0)
                return;
 
-       if (!function_graph_enter_ops(*parent, ip, fregs->fp, parent, gops))
+       if (!function_graph_enter_ops(*parent, ip, fp, (void *)fp, gops))
                *parent = (unsigned long)&return_to_handler;
 
        ftrace_test_recursion_unlock(bit);