[Crash-utility] x86 backtrace is dependent upon struct pt_regs at compile time

[Alan Tyson]

This problem has been reported before, but the discussion on it seemed
to move off track and I don't think that anyone really found the root cause.

The problem is that the x86 backtrace functionality in crash is
dependent upon the struct pt_regs taken from <asm/ptrace.h> at compile
time.  struct pt_regs changed in 2.6.20.  The result of this is that if
crash is compiled on 2.6.20 or later and subsequently used to look at a
2.6.19 or earlier dump, then exception frames are incorrectly displayed
and backtraces stop at them.

Here is an example of a 2.6.22-compiled crash displaying a trace from a
RHEL5 (2.6.18) dump:

crash> bt
PID: 3490   TASK: f7f5a000  CPU: 0   COMMAND: "insmod"
 #0 [f664ddd0] crash_kexec at c0441c78
 #1 [f664de14] die at c04064a4
 #2 [f664de44] do_page_fault at c0605eea
 #3 [f664de94] error_code (via page_fault) at c0405a6f
    EAX: 00000000  EBX: f8dd3400  ECX: 00200082  EDX: 00200000
    DS:  007b      ESI: f7bbeab0  ES:  007b      EDI: f7bbe800
    SS:  ffffe800      ESP: 00000000  EBP: f7bbead8
    CS:  0060      EIP: f8dd300d  ERR: ffffffff  EFLAGS: 00210296
crash>

Note that in the above, crash thinks that the exception frame is a user
mode one and not a kernel frame.

If crash was compiled on RHEL5 (2.6.18), then the trace looks like this:

crash> bt
PID: 3490   TASK: f7f5a000  CPU: 0   COMMAND: "insmod"
 #0 [f664ddd0] crash_kexec at c0441c78
 #1 [f664de14] die at c04064a4
 #2 [f664de44] do_page_fault at c0605eea
 #3 [f664de94] error_code (via page_fault) at c0405a6f
    EAX: 00000000  EBX: f8dd3400  ECX: 00200082  EDX: 00200000  EBP:
f7bbead8
    DS:  007b      ESI: f7bbeab0  ES:  007b      EDI: f7bbe800
    CS:  0060      EIP: f8dd300d  ERR: ffffffff  EFLAGS: 00210296
 #4 [f664dec8] function2 at f8dd300d
 #5 [f664dee0] sys_init_module at c043e717
 #6 [f664dfb8] system_call at c0404ef8
    EAX: ffffffda  EBX: 0861a028  ECX: 00010144  EDX: 0861a018
    DS:  007b      ESI: 00000000  ES:  007b      EDI: 00307ff4
    SS:  007b      ESP: bfe5695c  EBP: bfe569a8
    CS:  0073      EIP: 00d37402  ERR: 00000080  EFLAGS: 00200206
crash>

A similar problem happens if crash is compiled on pre-2.6.20 and then
used to analyse a 2.6.20 or later dump.


Dave, I have attached a patch to this e-mail which removes the
dependence upon <asm/prtrace.h> from lkcd_x86_trace.c (which is used for
non-LKCD dumps as well as LKCD dumps by the way).  I notice that
eframe_init() in x86.c initialises several variables which correspond to
the struct pt_regs so I've had to make these external for
lkcd_x86_trace.c's use.  I have no problem in this being reworked if you
feel that these symbols really should be in defs.h (or any other rework
that you think is fit, for that matter).

Regards,

Alan Tyson, HP.

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