libelf: Always call ftruncate before posix_fallocate to set the right size.

When elf_update.c (write_file) doesn't know the current maximum file length
it might have to reduce the file size. posix_fallocate can only extend the
file. So always call ftruncate before that to set the file size and making
sure the backing store is fully there. Add test cases for checking strip
in place (eu-strip without -o) actually reduces the file size. But only
for non-ET_REL files. We might not be able to strip ET_REL files (except
when they are kernel modules) because they might contain "dangling" symbol
table entries.

https://bugzilla.redhat.com/show_bug.cgi?id=1232206

Signed-off-by: Mark Wielaard <mjw@redhat.com>

2 files changed, 10 insertions, 2 deletions

diff --git a/libelf/ChangeLog b/libelf/ChangeLog
index 30017cd7..2d240074 100644
--- a/libelf/ChangeLog
+++ b/libelf/ChangeLog
@@ -1,3 +1,8 @@
+2015-06-16  Mark Wielaard  <mjw@redhat.com>
+
+	* elf_update.c (write_file): Always also use ftruncate before
+	posix_fallocate to make sure file has the right size.
+
 2015-06-04  Mark Wielaard  <mjw@redhat.com>
 
 	* elf_getdata.c (__libelf_type_aligns): Add entries for ELF_T_EHDR,
diff --git a/libelf/elf_update.c b/libelf/elf_update.c
index 9e34c466..9eb007bd 100644
--- a/libelf/elf_update.c
+++ b/libelf/elf_update.c
@@ -60,15 +60,18 @@ write_file (Elf *elf, off_t size, int change_bo, size_t shnum)
      new file.  We truncate the file later in this case.
 
      Note we use posix_fallocate to make sure the file content is really
-     there. Using ftruncate might mean the file is extended, but space
+     there. Only using ftruncate might mean the file is extended, but space
      isn't allocated yet. This might cause a SIGBUS once we write into
      the mmapped space and the disk is full. Using fallocate might fail
      on some file systems. posix_fallocate is required to extend the file
      and allocate enough space even if the underlying filesystem would
-     normally return EOPNOTSUPP.  */
+     normally return EOPNOTSUPP.  Note that we do also need to ftruncate
+     in case the maximum_size isn't known and the file needs to be shorter
+     because posix_fallocate can only extend.  */
   if (elf->parent == NULL
       && (elf->maximum_size == ~((size_t) 0)
 	  || (size_t) size > elf->maximum_size)
+      && unlikely (ftruncate (elf->fildes, size) != 0)
       && unlikely (posix_fallocate (elf->fildes, 0, size) != 0))
     {
       __libelf_seterrno (ELF_E_WRITE_ERROR);