gdb 'call' / 'print' commands don't work properly when attaching to R

[Kevin Ushey]

Sorry for the somewhat weird / vague title; hopefully the rest of this issue will make it more concrete ...

First, to set the stage; gdb and R both installed from apt repositories:

$ /usr/bin/gdb --version
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04) 9.2
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

$ /usr/bin/R --version
R version 4.0.5 (2021-03-31) -- "Shake and Throw"
Copyright (C) 2021 The R Foundation for Statistical Computing
Platform: x86_64-pc-linux-gnu (64-bit)

R is free software and comes with ABSOLUTELY NO WARRANTY.
You are welcome to redistribute it under the terms of the
GNU General Public License versions 2 or 3.
For more information about these matters see
https://www.gnu.org/licenses/.

Now, I can use gdb in batch mode to introspect a bash process; e.g. print the SHELL environment variable:

$ /usr/bin/gdb -batch -p $(pgrep -nx bash) -ex 'print (char*) getenv("SHELL")'
0x00007f5623d8c1db in __pselect (nfds=1, readfds=0x7ffdaeda52f0, writefds=0x0, exceptfds=0x0, timeout=<optimized out>, sigmask=0x5615fe778140 <_rl_orig_sigset>) at ../sysdeps/unix/sysv/linux/pselect.c:48
48 ../sysdeps/unix/sysv/linux/pselect.c: No such file or directory.
$1 = 0x5615ffc825d0 "/bin/bash"
[Inferior 1 (process 640102) detached]

But, if I try to do the same with a running R process, I see:

$ R -s -e "Sys.sleep(100)" &
[1] 643585

$ /usr/bin/gdb -batch -p $(pgrep -nx R) -ex 'print (char*) getenv("SHELL")'
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
0x00007f47944050da in select () from /usr/lib/x86_64-linux-gnu/libc.so.6
Invalid character '"' in expression.
[Inferior 1 (process 643585) detached]

Note the very confusing 'Invalid character '"' in expression.' error, implying that gdb was for some reason unable to handle the double-quoted string passed to getenv().

I cannot reproduce this in a version of gdb 9.2 built from sources locally; e.g.

$ gdb --version
GNU gdb (GDB) 9.2
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

$ gdb -batch -p $(pgrep -nx R) -ex 'print (char*) getenv("SHELL")'
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
0x00007fba31ad60da in select () from /usr/lib/x86_64-linux-gnu/libc.so.6
$1 = 0x7fff4dc9adfe "/bin/bash"
[Inferior 1 (process 701827) detached]

So it seems like something specifically is broken in the version of GDB packaged for Ubuntu 20.04.

[Kevin Ushey (kevinushey)]

In addition, I get a segfault if I use a single-quoted string rather than a double-quoted string. (My understanding is that single-quoted strings are normally used to reference symbols, so this should be an error but not a crash)

$ /usr/bin/gdb -batch -p $(pgrep -nx R) -ex "print (char*) getenv('oops')"
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
0x00007f69b4c870da in select () from /usr/lib/x86_64-linux-gnu/libc.so.6
Aborted (core dumped)