files-reg: fix buffer overflow on aarch64

[adrianreber]

Running the zdtm/static/unlink_regular00 test on Ubuntu 24.04 on aarch64 results in following error:

    # ./zdtm.py run -t zdtm/static/unlink_regular00 -k always
    userns is supported
    === Run 1/1 ================ zdtm/static/unlink_regular00
    ==================== Run zdtm/static/unlink_regular00 in ns ====================
    Skipping rtc at root
    Start test
    Test is SUID
    ./unlink_regular00 --pidfile=unlink_regular00.pid --outfile=unlink_regular00.out --dirname=unlink_regular00.test
    Run criu dump
    *** buffer overflow detected ***: terminated
    ############# Test zdtm/static/unlink_regular00 FAIL at CRIU dump ##############
    Test output: ================================

     <<< ================================
    Send the 9 signal to  47
    Wait for zdtm/static/unlink_regular00(47) to die for 0.100000
    ##################################### FAIL #####################################

According to the backtrace:

    #0  __pthread_kill_implementation (threadid=281473158467616, signo=signo@entry=6, no_tid=no_tid@entry=0) at ./nptl/pthread_kill.c:44
    #1  0x0000ffff93477690 in __pthread_kill_internal (signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:78
    #2  0x0000ffff9342cb3c in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
    #3  0x0000ffff93417e00 in __GI_abort () at ./stdlib/abort.c:79
    #4  0x0000ffff9346abf0 in __libc_message_impl (fmt=fmt@entry=0xffff93552a78 "*** %s ***: terminated\n") at ../sysdeps/posix/libc_fatal.c:132
    #5  0x0000ffff934e81a8 in __GI___fortify_fail (msg=msg@entry=0xffff93552a28 "buffer overflow detected") at ./debug/fortify_fail.c:24
    #6  0x0000ffff934e79e4 in __GI___chk_fail () at ./debug/chk_fail.c:28
    #7  0x0000ffff934e9070 in ___snprintf_chk (s=s@entry=0xffffc6ed04a3 "testfile", maxlen=maxlen@entry=4056, flag=flag@entry=2, slen=slen@entry=4053,
        format=format@entry=0xaaaacffe3888 "link_remap.%d") at ./debug/snprintf_chk.c:29
    #8  0x0000aaaacff4b8b8 in snprintf (__fmt=0xaaaacffe3888 "link_remap.%d", __n=4056, __s=0xffffc6ed04a3 "testfile")
        at /usr/include/aarch64-linux-gnu/bits/stdio2.h:54
    #9  create_link_remap (path=path@entry=0xffffc6ed2901 "/zdtm/static/unlink_regular00.test/subdir/testfile", len=len@entry=60, lfd=lfd@entry=20,
        idp=idp@entry=0xffffc6ed14ec, nsid=nsid@entry=0xaaaada2bac00, parms=parms@entry=0xffffc6ed2808, fallback=0xaaaacff4c6c0 <dump_linked_remap+96>,
        fallback@entry=0xffffc6ed2797) at criu/files-reg.c:1164
    #10 0x0000aaaacff4c6c0 in dump_linked_remap (path=path@entry=0xffffc6ed2901 "/zdtm/static/unlink_regular00.test/subdir/testfile", len=len@entry=60,
        parms=parms@entry=0xffffc6ed2808, lfd=lfd@entry=20, id=id@entry=12, nsid=nsid@entry=0xaaaada2bac00, fallback=fallback@entry=0xffffc6ed2797)
        at criu/files-reg.c:1198
    #11 0x0000aaaacff4d8b0 in check_path_remap (nsid=0xaaaada2bac00, id=12, lfd=20, parms=0xffffc6ed2808, link=<optimized out>) at criu/files-reg.c:1426
    #12 dump_one_reg_file (lfd=20, id=12, p=0xffffc6ed2808) at criu/files-reg.c:1827
    #13 0x0000aaaacff51078 in dump_one_file (pid=<optimized out>, fd=4, lfd=20, opts=opts@entry=0xaaaada2ba2c0, ctl=ctl@entry=0xaaaada2c4d50,
        e=e@entry=0xffffc6ed39c8, dfds=dfds@entry=0xaaaada2c3d40) at criu/files.c:581
    #14 0x0000aaaacff5176c in dump_task_files_seized (ctl=ctl@entry=0xaaaada2c4d50, item=item@entry=0xaaaada2b8f80, dfds=dfds@entry=0xaaaada2c3d40)
        at criu/files.c:657
    #15 0x0000aaaacff3d3c0 in dump_one_task (parent_ie=0x0, item=0xaaaada2b8f80) at criu/cr-dump.c:1679
    #16 cr_dump_tasks (pid=<optimized out>) at criu/cr-dump.c:2224
    #17 0x0000aaaacff163a0 in main (argc=<optimized out>, argv=0xffffc6ed40e8, envp=<optimized out>) at criu/crtools.c:293

This line is the problem:

    snprintf(tmp + 1, sizeof(link_name) - (size_t)(tmp - link_name - 1), "link_remap.%d", rfe.id);

The problem was that the -1 was on the inside of the braces and not on the outside. This way the destination size was increase by 1 instead of being decreased by 1 which triggered the the buffer overflow detection.