Swap file descriptors in running processes

fdswap.sh

#!/bin/bash
#
# fdswap
# Based on https://www.redpill-linpro.com/sysadvent/2015/12/04/changing-a-process-file-descriptor-with-gdb.html
# Alternatives https://github.com/bewakes/fdswap
# Updated by
#     anonymous friend of mhelsley
#     mhelsley
#         defanging, error handling, GDB verbosity/UI quirks, reduced appearance of magic numbers
#
if [ "$2" = "" ]; then
        echo "
    Usage: $0 /path/to/oldfile /path/to/newfile [pids]
    Example: $0 /var/log/daemon.log /var/log/newvolume/daemon.log 1234
    Example: $0 /dev/pts/53 /dev/null 2345"; exit 0
fi

if gdb --version > /dev/null 2>&1; then true
else echo "Unable to find gdb."; exit 1
fi

src="$1"; dst="$2"; shift; shift
pids=$*

# Set to non-empty to enable debugging this script
DEBUG_FDSWAP=""

# Hard-coded Linux open flags -- a more portable way to mask the flags would be to use a compiler
# but we know we're on Linux because we're going to be using /proc/$pid/fdinfo
O_TRUNC=$(( 01000 ))
O_CREAT=$(( 0100 ))
O_EXCL=$(( 0200 ))
O_DIRECTORY=$(( 0200000 ))
O_TMPFILE=$(( 020000000 | O_DIRECTORY ))
O_NOCTTY=$(( 0400 ))

FLMASK=$(( ~(O_CREAT|O_EXCL|O_TRUNC|O_TMPFILE) ))

for pid in ${pids:=$( /sbin/fuser $src | cut -d ':' -f 2 )};
do
    echo "src=$src, dst=$dst"
    echo "$src has $pid using it"
    (
    
        echo "set confirm off"
        echo "set pagination off"
        if [ -n "${DEBUG_FDSWAP}" ]; then
                echo "set trace-commands on"
                echo "set logging on"
        else
                echo "set width 0"
                echo "set height 0"
                echo "set verbose off"
        fi
        
        echo "attach $pid"
        #echo "\n\n"
        for ufd in $(LANG=C ls -l /proc/$pid/fd | \
                grep "$src"\$ | awk ' { print $9; } ');
                do
                offset=$(( $(grep -E '^pos:' /proc/$pid/fdinfo/$ufd | cut -d : -f 2) ))
                
                # The original fdswap script's flags, 66, correspond to 0102 or O_CREAT|O_RDWR.
                # Let's use the flags of the file descriptor we're replacing (e.g. O_RDONLY) but
                # limit them so we don't indirectly create anything new or clobber anything that
                # already exists.
                flags=$(( $(grep -E '^flags:' /proc/$pid/fdinfo/$ufd | cut -d : -f 2) ))
                (( flags &= FLMASK ))   # Don't create/clobber things in the filesystem
                (( flags |= O_NOCTTY )) # Don't change the process' controlling terminal

                # Emit GDB script snippet
                cat - <<-EOFOEOF
                define die
                    detach
                    quit
                end
                
                # Set mode to 0 so any created files have no permissions
                call open("$dst", $flags, 0)
                set \$tmpfd = \$
                die if \$tmpfd < 0
                # Only adjust the offset if we have to
                call(off64_t) lseek64(\$tmpfd, 0, 1) # Read current offset using SEEK_CUR
                if \$ != $offset
                    call(off64_t) lseek64(\$tmpfd, $offset, 0) # Set offset using SEEK_SET
                    die if \$ != $offset
                end
                call(int) dup2(\$tmpfd, $ufd)
                die if \$ != $ufd
                call close(\$tmpfd)
                EOFOEOF
        done
        echo 'detach'
        echo 'quit'
    ) | gdb -q -batch
done