When adding or changing data for sections, when there is also existing, non-dirty, sections then elf_update might write "fill" over some existing data.

This happens if the sections that aren't changed have some extra space between them. For example because they should be aligned. If two such sections are both not dirty then the last_position/offset written is not updated correctly. Only the data d_size of the existing data is added, missing any extra space needed for e.g. alignment. Then when dirty section data is later written too much "fill" is written because elf_update thinks there is a larger gap than there really is. Overwriting existing data.

Proposed patch that always updates the last_position/offset to the start of the section even if there is no changed/dirty section data:

diff --git a/libelf/elf32_updatefile.c b/libelf/elf32_updatefile.c
index 8dd85d1..7ac9951 100644
--- a/libelf/elf32_updatefile.c
+++ b/libelf/elf32_updatefile.c
@@ -343,9 +343,10 @@ __elfw2(LIBELFBITS,updatemmap) (Elf *elf, int change_bo, size_t shnum)
                  {
                    fill_mmap (dl->data.d.d_off, last_position, scn_start,
                               shdr_start, shdr_end);
-                   last_position = scn_start + dl->data.d.d_off;
                  }
 
+               last_position = scn_start + dl->data.d.d_off;
+
                if ((scn->flags | dl->flags | elf->flags) & ELF_F_DIRTY)
                  {
                    /* Let it go backward if the sections use a bogus
@@ -353,8 +354,6 @@ __elfw2(LIBELFBITS,updatemmap) (Elf *elf, int change_bo, size_t shnum)
                       user's section data with the latest one, rather than
                       crashing.  */
 
-                   last_position = scn_start + dl->data.d.d_off;
-
                    if (unlikely (change_bo))
                      {
 #if EV_NUM != 2
@@ -728,6 +727,8 @@ __elfw2(LIBELFBITS,updatefile) (Elf *elf, int change_bo, size_t shnum)
                      }
                  }
 
+               last_offset = scn_start + dl->data.d.d_off;
+
                if ((scn->flags | dl->flags | elf->flags) & ELF_F_DIRTY)
                  {
                    char tmpbuf[MAX_TMPBUF];
@@ -738,8 +739,6 @@ __elfw2(LIBELFBITS,updatefile) (Elf *elf, int change_bo, size_t shnum)
                       user's section data with the latest one, rather than
                       crashing.  */
 
-                   last_offset = scn_start + dl->data.d.d_off;
-
                    if (unlikely (change_bo))
                      {
 #if EV_NUM != 2