[Crash-utility] crash version 4.0-4.1 is available

[Vivek Goyal]

On Fri, Apr 27, 2007 at 12:12:29PM -0400, Dave Anderson wrote:
> 
> - Implemented dependable backtraces for the x86_64 architecture. (!!!)
>   This feature builds upon the current "low_budget" backtrace function,
>   and also required the fix for the BUG()/ud2a disassembly problem
>   addressed in 4.0-3.22.  It does not require kernel unwind support,
>   but rather it calculates function framesizes by disassembling the
>   code from the beginning of the function to the point where it calls
>   the next function, parsing for add or sub instructions on the rsp,
>   and for push and pop instructions, thereby determining the framesize
>   of the function at the point of the call.  This is similar to what is
>   done for x86, but requires far less hackery.  You will notice a slight
>   hitch the first time a "bt" is done on a task, but for each text
>   return address in any backtrace, its framesize is cached for all
>   subsequent instances.  It also accounts for backtrace text return
>   addresses from the .text.lock section, by appending "(via function)"
>   to the end of the frame line.  Also, because it layers on top of the
>   current backtrace code, it does not compromise the capability of
>   switching between the process, IRQ, and exception stacks.  That all
>   being said, 100% accuracy cannot be guaranteed.  But for the ~30
>   sample dumpfiles I keep around for x86_64 testing, I cannot find any
>   obviously invalid backtraces.  However, if there is any doubt, the
>   "bt -o" option will perform backtraces using the "old" manner; and
>   "bt -O" will force the old manner to always be used.  Of course the
>   "bt -t" and "bt -T" options are still available.  It's interesting to
>   redirect the output of "foreach bt" to a file using this version, and
>   then compare it with the output from an older version.
>   (anderson at redhat.com)
> 

Hi Dave,

Why to go for this approach? Does it give better backtraces as compared
to retrieving the information from kernel vmlinux (unwind info) or
from debug sections? 

Thanks
Vivek