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

Gary Benson
2016-02-12T10:31:49Z

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?

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

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

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.

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

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