GCC 5.1.1 with -O2 and -fsanitize=address reports
the following from `chmod a+rx ..` for example:
ERROR: AddressSanitizer: heap-buffer-overflow on address 0x61200000be48
READ of size 4 at 0x61200000be48 thread T0
#0 0x40f0a1 in fts_stat lib/fts.c:1821
#1 0x4141ec in fts_open lib/fts.c:514
#2 0x40ca6e in xfts_open lib/xfts.c:36
#3 0x402c35 in process_files src/chmod.c:336
#4 0x402c35 in main src/chmod.c:566
#5 0x7f8fdc38678f in __libc_start_main (/lib64/libc.so.6+0x2078f)
#6 0x404608 in _start (/home/padraig/git/coreutils/src/chmod+0x404608)
0x61200000be4b is located 0 bytes to the right of
267-byte region [0x61200000bd40,0x61200000be4b) allocated by thread T0 here:
#0 0x7f8fdd4cfa0a in malloc (/lib64/libasan.so.2+0x98a0a)
#1 0x40f22c in fts_alloc lib/fts.c:1916
The read of size 4 from a heap object of size 3 is indeed invalid,
though this may be due to incorrect padding assumptions by GCC, see:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66661
The read is coming from the a[2] access in ISDOT() which GCC seems
to assume can access according to the alignment of the main structure,
since the flexible array members are being accessed directly.
One could cast the parameter to ISDOT() to (char const*) to restrict
to byte at a time access, however it seems more correct and maintainable
to increase the buffer size to the appropriate alignment to avoid this issue.
For reference some notes on alignment and flexible array members are at:
https://sites.google.com/site/embeddedmonologue/home/c-programming/data-alignment-structure-padding-and-flexible-array-member
* lib/fts.c (fts_alloc): Increase allocation to alignof(FTSENT).
* modules/fts: Depend on stdalign.