libelf: Only use posix_fallocate when using mmap. Ignore unsupported errors.

Don't use posix_fallocate when not using mmap. It is enough to ftruncate
the file to the right size. pwrite will report an error if there is no
disk space left. And on file systems that don't support fallocate it
might duplicate writes in that case. When using posix_fallocate do ignore
most errors. Other libc implementations don't guarantee the call actually
works always and even with glibc there might be an unexpected error from
the fallback code when the file system doesn't support fallocate. That is
pretty nasty since we might get a SIGBUS in that case when writing to the
mmapped memory. But the chance of that happening is very small.  And will
normally never happen with glibc. So only report an error when
posix_fallocate reports ENOSPC.

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

2 files changed, 28 insertions, 13 deletions

diff --git a/libelf/ChangeLog b/libelf/ChangeLog
index b1d1ecdf..af2438b4 100644
--- a/libelf/ChangeLog
+++ b/libelf/ChangeLog
@@ -1,3 +1,8 @@
+2015-10-05  Mark Wielaard  <mjw@redhat.com>
+
+	* elf_update.c (write_file): Only use posix_fallocate when using
+	mmap. Only report failure when errno is ENOSPC.
+
 2015-10-09  Josh Stone  <jistone@redhat.com>
 
 	* libelfP.h (struct Elf): Replace off64_t with off_t.
diff --git a/libelf/elf_update.c b/libelf/elf_update.c
index 00f7a010..c635eb32 100644
--- a/libelf/elf_update.c
+++ b/libelf/elf_update.c
@@ -57,22 +57,11 @@ write_file (Elf *elf, off_t size, int change_bo, size_t shnum)
      We cannot do this if this file is in an archive.  We also don't
      do it *now* if we are shortening the file since this would
      prevent programs to use the data of the file in generating the
-     new file.  We truncate the file later in this case.
-
-     Note we use posix_fallocate to make sure the file content is really
-     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.  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.  */
+     new file.  We truncate the file later in this case.  */
   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))
+      && unlikely (ftruncate (elf->fildes, size) != 0))
     {
       __libelf_seterrno (ELF_E_WRITE_ERROR);
       return -1;
@@ -89,6 +78,27 @@ write_file (Elf *elf, off_t size, int change_bo, size_t shnum)
 
   if (elf->map_address != NULL)
     {
+      /* When using mmap we want to make sure the file content is
+	 really 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.  In glibc posix_fallocate is required to extend the
+	 file and allocate enough space even if the underlying
+	 filesystem would normally return EOPNOTSUPP.  But other
+	 implementations might not work as expected.  And the glibc
+	 fallback case might fail (with unexpected errnos) in some cases.
+	 So we only report an error when the call fails and errno is
+	 ENOSPC. Otherwise we ignore the error and treat it as just hint.  */
+      if (elf->parent == NULL
+	  && (elf->maximum_size == ~((size_t) 0)
+	      || (size_t) size > elf->maximum_size)
+	  && unlikely (posix_fallocate (elf->fildes, 0, size) != 0))
+	if (errno == ENOSPC)
+	  {
+	    __libelf_seterrno (ELF_E_WRITE_ERROR);
+	    return -1;
+	  }
+
       /* The file is mmaped.  */
       if ((class == ELFCLASS32
 	   ? __elf32_updatemmap (elf, change_bo, shnum)