tdep/2029: rs6000/powerpc stack backtrace bugs (skip_prologue)

[strauman at slac dot stanford dot edu]

>Number:         2029
>Category:       tdep
>Synopsis:       rs6000/powerpc stack backtrace bugs (skip_prologue)
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    unassigned
>State:          open
>Class:          patch
>Submitter-Id:   net
>Arrival-Date:   Tue Nov 08 02:08:02 GMT 2005
>Closed-Date:
>Last-Modified:
>Originator:     Till Straumann
>Release:        6.3
>Organization:
>Environment:
powerpc-linux; powerpc-rtems
>Description:
There are problems with gdb's stack backtrace sometimes not working properly (powerpc target). The backtrace gets into what seems an endless loop that is terminated with the message:
"Previous frame inner to this frame (corrupt stack?)"

a) in some cases, code compiled with '-fpic/PIC' causes problems
b) in some cases, code generated with more recent gcc does.
>How-To-Repeat:
a) for '-fpic' see http://sourceware.org/ml/gdb/2004-11/msg00060.html

for b) use gdb on itself (compiled with gcc 3.3.5 (Debian 1:3.3.5-13)), set breakpoint in skip_prologue
and try a backtrace ;-)

gdb gdb
GNU gdb 6.3
...
(gdb) set prompt (gdb-mom)
(gdb-mom)break skip_prologue
Breakpoint 1 at 0x1006668c: file ../../gdb/rs6000-tdep.c, line 848.
(gdb-mom)run
GNU gdb 6.3
....
(gdb) file gdb
(gdb) break main

Breakpoint 1, skip_prologue (pc=275643600, lim_pc=275655304, fdata=0x10430000) at ../../gdb/rs6000-tdep.c:848

848       const struct bfd_arch_info *arch_info = gdbarch_bfd_arch_info (current_gdbarch);
(gdb-mom) bt
#0  skip_prologue (pc=275643600, lim_pc=275655304, fdata=0x10430000) at ../../gdb/rs6000-tdep.c:848
#1  0x1006607c in rs6000_skip_prologue (pc=268756968) at ../../gdb/rs6000-tdep.c:499
#2  0x1010bcf8 in gdbarch_skip_prologue (gdbarch=0x0, ip=0) at ../../gdb/gdbarch.c:2902
#3  0x1010bcf8 in gdbarch_skip_prologue (gdbarch=0x0, ip=0) at ../../gdb/gdbarch.c:2902
#4  0x1010bcf8 in gdbarch_skip_prologue (gdbarch=0x0, ip=0) at ../../gdb/gdbarch.c:2902
#5  0x1010bcf8 in gdbarch_skip_prologue (gdbarch=0x0, ip=0) at ../../gdb/gdbarch.c:2902
#6  0x1010bcf8 in gdbarch_skip_prologue (gdbarch=0x0, ip=0) at ../../gdb/gdbarch.c:2902
#7  0x1010bcf8 in gdbarch_skip_prologue (gdbarch=0x0, ip=0) at ../../gdb/gdbarch.c:2902
...
Previous frame inner to this frame (corrupt stack?)
(gdb-mom)
>Fix:
a) -fpic uses a 'bcl' instruction to read the contents of LR which is not recognized by 'skip_prologue'.
b) the code forcing 'skip_prologue' to scan beyond unrecognized instructions is wrong causing the scan to be aborted prematurely (recent gcc moves other instructions into the prologue more aggressively).

Proposed fixes can be found (along with some more detailed explanations) in the attached 'diff-file'.
>Release-Note:
>Audit-Trail:
>Unformatted:
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Content-Type: text/x-patch; name="rs6000.diff"
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Content-Disposition: attachment; filename="rs6000.diff"

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