Re: Revamp sniffer; Was: [obish?sym;rfa:doc] Wire up vsyscall

Andrew Cagney
2004-06-15T20:17:14Z

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Ok,

I just figured out why amd64 GNU/Linux signal trampolines don't work
correctly and the reason is scary!

After signal delivery (e.x., sigaltstack testcase) the stack looks like:

sentinel: SENTINEL_FRAME
catcher: NORMAL_FRAME, dwarf2 unwinder
<sigtramp>: SIGTRAMP_FRAME, sigtramp unwinder
thrower: NORMAL_FRAME, dwarf2 unwinder

while later after catcher has "finish"ed it looks like:

sentinel: SENTINEL_FRAME
__restore_rt: NORMAL_FRAME, dwarf2 unwinder
<corrupt stack>

Notice how the first time round the trampoline was correctly identified
but not the second. What happened?

In the first case:

- linux jumps to catcher setting it's return address to <sigtramp>'s
entry point
- gdb correctly identifies catcher as a callee or normal frame
- gdb goes to unwind catcher and starts sniffing <sigtramp>
- the dwarf2 sniffer, when sniffing <sigtramp>, asks for that frame's
address-in-block, since the catcher is a normal frame, that translates
to <catcher's-return-address>-1
- <catcher's-return-address>-1 translates into <sigtramp>-1
- since <sigtramp>-1 doesn't have any CFI (lucky eh?) dwarf2 dismisses
the frame
- the sigtramp unwinder gets to take a wiff and identifies the frame

while in the second case (no catcher):

- the dwarf2 sniffer, when sniffing <sigtramp>, asks for that frame's
address-in-block, since the next frame is a sentinel that translates
into <catcher's-return-address>
- <catcher's-return-address> translates into <sigtramp>
- <sigtramp> does have CFI so dwarf2 claims the frame marking it
"normal", outch!

So for the moment, I think the sigtramp frame should always be sniffed
before the dwarf2 frame. We can revisit the possability of modifing
dwarf2 later, much later.

Andrew

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ping

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Mark,

Want do you want to do here? Commit daniel's patch for the short term
while I clean this up proper over the medium term (appending it to the
vsyscall list)?

Andrew

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From: Andrew Cagney <cagney@gnu.org>
To: Mark Kettenis <kettenis@chello.nl>
Cc: drow@false.org, gdb-patches@sources.redhat.com, roland@redhat.com
Subject: Revamp sniffer; Was: [obish?sym;rfa:doc] Wire up vsyscall
Date: Mon, 17 May 2004 13:14:48 -0400
Message-ID: <40A8F308.9040900@gnu.org>

> 2004-01-25 Daniel Jacobowitz <drow@mvista.com>
>
> * i386-tdep.c (i386_gdbarch_init): Check for signal frame first.
> * i386-linux-tdep.c (i386_linux_pc_in_sigtramp): Handle
> __kernel_sigreturn.
>
[...]
> I'm somewhat reluctant to throw the CFI completely overboard.

Yes.

> How
> about extending the DWARF2 unwinder such that we can set the
> [SIGTRAMP_FRAME] attribute based on the function's name?

The frame attributes (tramp et.al.) in many cases are orthogonal to the
unwind mechanism, but sometimes they are not).

At present the frame sniffer is focused solely on identifying and
returning an unwinder. The frame's type being identified as an
unexpected sid effect. I think this should be inverted vis:

frame_sniffer () ->
- attributes
- unwinder (or NULL, indicating sniff for unwinder)
- base (or NULL, indicating sniff for base)
- ...

so while the type is determined up front, other things are optional.
That way here the tramp code would look like:

if (name == _kernel_sigreturn)
if (cfi at address)
return { attribute=TRAMP; unwinder=cfi_unwinder; base=NULL };
else
return { attribute=TRAMP; unwinder=alt_unwinder; base=NULL };

this also opens the way for us doing:

if (name == solib-trampoline)
return { attribute=TRAMP|CALLEE; unwinder=NULL; base=NULL }

i.e., it's a trampoline (need to skip it) but let something more generic
identify the unwinder.

Andrew