On Mon, 8 Jan 2024 15:03:21 +0000
Mark Rutland <mark.rutland@xxxxxxx> wrote:
> 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.
Yeah, that will be better to keep the line short.
Thank you,
>
> 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);
>
--
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
