[Bug libdw/28658] New: UBSan seems to report a divison-by-zero in dwfl_link_map_report

[evvers at ya dot ru]

https://sourceware.org/bugzilla/show_bug.cgi?id=28658

            Bug ID: 28658
           Summary: UBSan seems to report a divison-by-zero in
                    dwfl_link_map_report
           Product: elfutils
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libdw
          Assignee: unassigned at sourceware dot org
          Reporter: evvers at ya dot ru
                CC: elfutils-devel at sourceware dot org
  Target Milestone: ---

One of systemd fuzz target run under UBSan discovered a "divison-by-zero". It
can be reproduced by building the master branch of the elfutils repository with
UBSan and passing the attachment to `./src/stack`
```
$ git describe
elfutils-0.186-7-g99782bd2

autoreconf -i -f
./configure --enable-maintainer-mode CFLAGS='-g -O1 -fno-omit-frame-pointer'
--enable-sanitize-undefined CXXFLAGS='-g -O1 -fno-omit-frame-pointer'
make -j$(nproc) V=1
LD_PRELOAD="./libelf/libelf.so ./libdw/libdw.so"
UBSAN_OPTIONS=print_stacktrace=1:print_summary=1  ./src/stack --core
../oss-fuzz-41574
link_map.c:873:12: runtime error: division by zero
    #0 0x7f93972cf29a in dwfl_link_map_report
/home/vagrant/elfutils/libdwfl/link_map.c:873
    #1 0x7f93972d25c7 in _new.dwfl_core_file_report
/home/vagrant/elfutils/libdwfl/core-file.c:548
    #2 0x402b0f in parse_opt /home/vagrant/elfutils/src/stack.c:595
    #3 0x7f939654df81 in __argp_parse (/lib64/libc.so.6+0x10cf81)
    #4 0x403d98 in main /home/vagrant/elfutils/src/stack.c:695
    #5 0x7f9396468b74 in __libc_start_main (/lib64/libc.so.6+0x27b74)
    #6 0x4024cd in _start (/home/vagrant/elfutils/src/stack+0x4024cd)

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior link_map.c:873:12 in
```
It was also reported in
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=41574
```
        Running:
/mnt/scratch0/clusterfuzz/bot/inputs/fuzzer-testcases/crash-747fcf4f5bf93741e8fc31e9cbb44a70865e4c07
link_map.c:873:12: runtime error: division by zero
    #0 0x51cae6 in dwfl_link_map_report /src/elfutils/libdwfl/link_map.c:873:12
    #1 0x4b868b in dwfl_core_file_report
/src/elfutils/libdwfl/core-file.c:548:16
    #2 0x4b363e in LLVMFuzzerTestOneInput /src/fuzz-dwfl-core.c:52:6
    #3 0x43f1b3 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*,
unsigned long) cxa_noexception.cpp:0
    #4 0x42aac2 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned
long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:324:6
    #5 0x43058a in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char
const*, unsigned long)) cxa_noexception.cpp:0
    #6 0x4594b2 in main
/src/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
    #7 0x7f2bedb160b2 in __libc_start_main
/build/glibc-eX1tMB/glibc-2.31/csu/libc-start.c:308:16
    #8 0x407d4d in _start
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior link_map.c:873:12 in
```

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