[PATCH] libelf: Only use posix_fallocate when using mmap. Ignore unsupported errno.

[Mark Wielaard <mjw at redhat dot com>]

When not using mmap it is enough the just 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. When
using posix_fallocate do ignore errors indicating the file system doesn't
support fallocate. These will never happen with glibc, but might on some
other implementations. 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 people using such a libc implementation are
on their own.

Signed-off-by: Mark Wielaard <mjw@redhat.com>
---
 libelf/ChangeLog    |  5 +++++
 libelf/elf_update.c | 35 ++++++++++++++++++++++-------------
 2 files changed, 27 insertions(+), 13 deletions(-)

diff --git a/libelf/ChangeLog b/libelf/ChangeLog
index 1916877..8a08c5f 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. Don't fail on EINVAL or EOPNOTSUPP.
+
 2015-09-23  Mark Wielaard  <mjw@redhat.com>
 
 	* elf32_getehdr.c (getehdr_wrlock): Mark as internal_function.
diff --git a/libelf/elf_update.c b/libelf/elf_update.c
index 00f7a01..3d27f7f 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 +78,26 @@ 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.  So we ignore
+	 EOPNOTSUPP and EINVAL, which posix specifies as alternative,
+	 errors here.  */
+      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 != EINVAL && errno != EOPNOTSUPP)
+	  {
+	    __libelf_seterrno (ELF_E_WRITE_ERROR);
+	    return -1;
+	  }
+
       /* The file is mmaped.  */
       if ((class == ELFCLASS32
 	   ? __elf32_updatemmap (elf, change_bo, shnum)
-- 
1.8.3.1