Created attachment 380957 [details]
testcase: libtar-main.c

Description of problem:
When executing a testcase for a toy app, I started to write, valgrind observed invalid memory access.

Version-Release number of selected component (if applicable):
$ rpm -q libtar{,-devel,-debuginfo} valgrind{,-devel}
libtar-1.2.11-15.fc12.i686
libtar-devel-1.2.11-15.fc12.i686
libtar-debuginfo-1.2.11-15.fc12.i686
valgrind-3.5.0-9.i686
valgrind-devel-3.5.0-9.i686

How reproducible:
Compile and valgrind the test case.

Steps to Reproduce:
1. gcc -o libtar-test libtar-main.c -ltar
2. valgrind --exit-errorcode=1 -q ./libtar-test
  
Actual results:
FAIL FOLLOWS:
==9338== Conditional jump or move depends on uninitialised value(s)
==9338==    at 0xBC3683: th_set_path (encode.c:85)
==9338==    by 0x8048890: main (libtar-main.c:60)
==9338== 

Expected results:
Silence and a happy valgrind.

Additional info:
This issue happens because of the TH_ISDIR() macro used in th_set_path(). It is defined like this:
> #define TH_ISDIR(t)     ((t)->th_buf.typeflag == DIRTYPE \
>                          || S_ISDIR((mode_t)oct_to_int((t)->th_buf.mode)) \
>                          || ((t)->th_buf.typeflag == AREGTYPE \
>                              && ((t)->th_buf.name[strlen((t)->th_buf.name) - 1] == '/')))

Now when we use an empty-initialized header, the name is an array of 100 '\0' bytes => strlen((t)->th_buf.name) == 0 => strlen((t)->th_buf.name) - 1 == -1 => (t)->th_buf.name[-1] == <last byte before the tar header>.

I'm not sure this behavior is an exploitable security issue. It's at least a major defect in that library preventing it from being used in properly valgrinded test cases.

Will provide a patch.