Deadlock from --enable-thread-safety

[Heather McIntyre <hsm2@rice.edu>]

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I've been making --enable-thread-safety (USE_LOCKS) more viable by fixing
deadlocks throughout the libelf library. This has required minimal code
changes so far. However, I've hit a snag where "__elf64_updatenull_wrlock"
calls "elf64_getchdr," which leads to "elf64_getshdr." This sequence
attempts to acquire a read lock under a write lock and fails. Similarly,
"elf64_getchdr" calls "elf_getdata," which also tries and fails to
implement a read lock.

Unfortunately, fixing this particular deadlock requires more than minimal
code changes. From what I understand, I may have a few options on how to
fix this:

1) Create copies of the getchdr and elf_getdata functions, renaming them
getchdr_wrlock and elf_getdata_wrlock. However, multiple copies of these
functions could bloat the code, which may not be ideal.
2) Some type of write lock flag to indicate when a write lock is active.
Either within the locking macro in eu-config.h or passed as an argument in
the functions.

Ultimately, I thought others may want to look into this to see if there
might be options for a better solution. Here is the backtrace:

Program received signal SIGABRT, Aborted.
0x00007ffff7837aff in raise () from /lib64/libc.so.6
#0  0x00007ffff7837aff in raise () from /lib64/libc.so.6
#1  0x00007ffff780aea5 in abort () from /lib64/libc.so.6
#2  0x00007ffff780ad79 in __assert_fail_base.cold.0 () from /lib64/libc.so.6
#3  0x00007ffff78304c9 in __assert_perror_fail () from /lib64/libc.so.6
#4  0x00007ffff7fda554 in elf64_getshdr (scn=0x40fc20) at
../../libelf/elf32_getshdr.c:292
#5  0x00007ffff7fe9590 in elf64_getchdr (scn=0x40fc20) at
../../libelf/elf32_getchdr.c:45
#6  0x00007ffff7fdf690 in __elf64_updatenull_wrlock (elf=0x40d880,
change_bop=0x7fffffffac60, shnum=30) at ../../libelf/elf32_updatenull.c:407
#7  0x00007ffff7fdd83d in elf_update (elf=0x40d880, cmd=ELF_C_WRITE) at
../../libelf/elf_update.c:211
#8  0x0000000000405d9e in process_file (fname=0x7fffffffbb96
"testfile-s390x-hash-both") at ../../src/elfcompress.c:1336
#9  0x0000000000406232 in main (argc=7, argv=0x7fffffffb768) at
../../src/elfcompress.c:1458