Re: [PATCH] Remote debugging without a binary (regression)

Luis Machado
2016-02-12T10:59:48Z

On 02/12/2016 08:31 AM, Gary Benson wrote:
> Luis Machado wrote:
>> On 02/11/2016 02:35 PM, Gary Benson wrote:
>>> Luis Machado wrote:
>>>> It looks like this is fallout from the changes that were added to
>>>> make GDB a bit smarter about locating the binary that is being
>>>> debugged.
>>>>
>>>> When one attempts to do gdbserver-based debugging in the same
>>>> machine/filesystem, there is no problem at all.
>>>>
>>>> If the user wants to have the files transfered over the wire, GDB
>>>> will handle it. If the user sets a local sysroot path and doesn't
>>>> want the file coming through the wire, GDB will use process
>>>> information to attempt to locate the binary in the local filesystem.
>>>>
>>>> Now, considering we have a GDB instance running on a local machine
>>>> and a gdbserver instance running on a remote machine with a
>>>> completely separate filesystem, having the sysroot set will prevent
>>>> the file from being downloaded.
>>>>
>>>> GDB will then attempt to be smart and locate the binary through the
>>>> path that is reported by gdbserver. This path is from the remote
>>>> filesystem though, so there is a chance this file won't even exist
>>>> in the local filesystem.
>>>>
>>>> In a normal native session (where we start the process from scratch)
>>>> this would result in a "No such file or directory" error. And that
>>>> is fine, because we really need a binary to get the process started.
>>>>
>>>> But with a local GDB plus a remote gdbserver on a different
>>>> filesystem, we will see the same error and the debugging session
>>>> will end abruptly, giving the user no chance of doing some debugging
>>>> without a symbol file.
>>>>
>>>> --
>>>> Remote debugging using some_machine:12345
>>>> <some_remote_filesystem_path/gdb.d/gdb.base/break: No such file or directory.
>>>> --
>>>>
>>>> I tracked this down to remote_add_inferior and its call to (mainly)
>>>> exec_file_locate_attach. This specific function will call other
>>>> functions that may throw an error, causing everything to stop dead
>>>> on its tracks.
>>>>
>>>> The following patch guards such a call to prevent those errors from
>>>> disrupting a potential debugging session, and display only a warning.
>>>>
>>>> --
>>>> Remote debugging using some_machine:12345
>>>> warning: <some_remote_filesystem_path/gdb.d/gdb.base/break: No such file or directory.
>>>> --
>>>>
>>>> I tried to come up with a valid testcase that would fail with a
>>>> local gdb/gdbserver combination, but it seems GDB is smart enough to
>>>> recognize a deleted binary with the help of /proc, thus foiling my
>>>> attempts.
>>>
>>> I don't have any fundamental objection to your patch but I'm not
>>> really sure I understand what's going on here. You have the
>>> sysroot set to some path that does not exist? What are you trying
>>> to do and what are you expecting to be able to do? What did GDB
>>> do before?
>>
>> No. The sysroot being anything other than "target:" is needed is
>> order to prevent gdbserver from transfering the files over (too
>> slow). Plus, i'm not loading any symbol file on GDB's side.
>>
>> So i'm trying to connect to a gdbserver running on a remote system
>> with a separate filesystem. gdbserver will now report the full path
>> to the binary on the remote end via the new qXfer:exec-file packet,
>> even if i force the sysroot to be empty.
>>
>> In summary, GDB (running on a local machine) is attempting to use
>> that path provided by qXfer:exec-file to open a symbol file that
>> only exists on the remote end's filesystem, not in the local
>> filesystem where GDB is running.
>>
>> If GDB fails to locate that file, it will drop the connection due to
>> a error that is thrown from within exec_file_locate_attach and its
>> callees.
>>
>> The correct behavior is for GDB to ignore the lack of a symbol file
>> and carry on connecting to the remote target, allowing a symbol-less
>> debugging session.
>>
>> Does that make it clear?
>
> I'm getting there, but I have a couple more questions:
>
> 1) What exactly are you setting sysroot to? Is it:
> - the empty string
> - a directory full of shared libraries but not the main executable
> - an empty directory
> - a non-existent directory?
>

It doesn't matter, as long as it is not "target:", meaning we really
don't want to load files using the help of gdbserver.

> 2) What exactly is the error being thrown within exec_file_locate_attach?
>

The one i mentioned above:

<some_remote_filesystem_path>/gdb.d/gdb.base/break: No such file or
directory.

This comes from exec_file_attach. In my specific case above, this is
thrown from gdb/exec.c:268, a call to perror_with_name.

if (scratch_chan < 0)
perror_with_name (filename);

It seems this was introduced with commit
1b6e6f5c7ffba559a681d11852acf38ef48dceff, with the addition of a call to
exec_file_locate_attach from within remote_add_inferior.

That unguarded call to exec_file_locate_attach doesn't look safe since
its callees can throw errors and potentially disrupt a connection attempt.

> FWIW I tried this (both on the same machine):
>

Doing so with both on the same machine/filesystem will not reproduce the
problem, as i mentioned in my original post. GDB and gdbserver need to
be on separate filesystems.

> gdbserver :9999 /bin/ls
> gdb -q -ex "set sysroot /whatever" -ex "target remote :9999"
>
> and got this:
>
> Reading symbols from /bin/ls...(no debugging symbols found)...done.
>
> which I think is an error: the sysroot is being ignored.
>

Isn't it being ignored because GDB managed to figure out the path and
successfully open the symbol file?

If it should honor the sysroot in that case, that looks like a different
problem than the one i was originally chasing. There may be more bugs. :-)

> Once again, I have no fundamental problem with your patch, but I want
> to make sure we're not papering over some deeper issue.
>
> Thanks,
> Gary
>