Re: [RFC PATCH] Add a new 'info proc time' subcommand of 'info proc'.

[John Baldwin <jhb@FreeBSD.org>]

On 7/28/21 8:06 AM, Rustam Kovhaev wrote:
> Hi John,
> 
> On Wed, Jul 28, 2021 at 08:00:28AM -0700, John Baldwin wrote:
>> On 7/27/21 3:08 PM, Rustam Kovhaev via Gdb-patches wrote:
>>> Sometimes (unfortunately) I have to review windows user-space cores in
>>> windbg, and there is one feature that I really want to implement in gdb,
>>> but I don't know whether it is a good idea or not, and why it has not
>>> yet been implemented in gdb.
>>>
>>> In wdbg there is a .time command that gives me time when core was taken.
>>> I could not find the same functionality in gdb and in elf core.
>>> I know about kernel core_pattern and timestamp, and there are user-space
>>> daemons that write the timestamp, and sometimes if I am lucky I can get
>>> timestamp from modified/created file attributes and this solves the
>>> problem most of the time, but quite often I get only core.PID file +
>>> some app log and there is no way for me to figure out when exactly the
>>> core was taken.
>>>
>>> Current patch does not take into account lots of things like endianness,
>>> cpu archs other than x86, other code paths, etc, and there is also
>>> kernel side to modify and coordinate, but it does work in my lab, and I
>>> was pretty happy to learn a little bit about the project.
>>
>> Does the Linux kernel write out NT_TIME notes when creating core dumps
>> or would this command only work on cores generated by a patched gdb's
>> 'gcore' command?
> 
> It does not. Linux kernel will need to be patched too, but before
> sending a kernel RFC patch I wanted to run this proposal by gdb
> mainteners.

I suspect that in general it's probably best to coordinate so that the
kernel agrees to the value of NT_TIME, etc.  I work on FreeBSD myself
rather than Linux, but I wouldn't want to commit something to gdb to
support a new note unless the specific ABI (e.g. note layout, NT_ value)
was confirmed with the kernel folks as well.  Even if the Linux kernel
doesn't adopt the note, it needs to agree to reserving the value for
NT_TIME so it doesn't get reused in the future for something else.

Also, changes to binutils and bfd need to be sent to binutils@ for their
review.

-- 
John Baldwin