Handle merged strtab/shstrtab string tables in strip and unstrip.

ELF files can share the section header string table (e_shstrndx) with
the symtab .strtab section. That might in some cases save a bit of space
since symbols and sections might share some (sub)strings. To handle that
eu-strip just needs to not unconditionally remove the .shstrtab section
(it will be properly marked as used/unused as needed). eu-unstrip needs
to make sure the section names are added to the strtab if it decides to
rewrite that section. Also makes sure that eu-strip won't move around
a SHT_NOBITS section that has SHF_ALLOC set. Although it is allowed to
move such sections around, there is no benefit. And some tools might
expect no allocated section to move around, not even a nobits section.
It also makes it harder to do "roundtripping" sanity checks that make
sure splitting a file with eu-strip and then reconstructed with eu-unstrip
produce the same ELF file (as is done in the new run-strip-strmerge.sh).

Introduces a somewhat large test generator elfstrmerge.c that will
hopefully turn into a more generic string table merger program.

Signed-off-by: Mark Wielaard <[email protected]>

1 files changed, 9 insertions, 6 deletions

diff --git a/src/strip.c b/src/strip.c
index 8b08d72d..41169eda 100644
--- a/src/strip.c
+++ b/src/strip.c
@@ -644,10 +644,12 @@ handle_elf (int fd, Elf *elf, const char *prefix, const char *fname,
 	goto illformed;
 
       /* Sections in files other than relocatable object files which
-	 don't contain any file content or are not loaded can be freely
-	 moved by us.  In relocatable object files everything can be moved.  */
+	 not loaded can be freely moved by us.  In theory we can also
+	 freely move around allocated nobits sections.  But we don't
+	 to keep the layout of all allocated sections as similar as
+	 possible to the original file.  In relocatable object files
+	 everything can be moved.  */
       if (ehdr->e_type == ET_REL
-	  || shdr_info[cnt].shdr.sh_type == SHT_NOBITS
 	  || (shdr_info[cnt].shdr.sh_flags & SHF_ALLOC) == 0)
 	shdr_info[cnt].shdr.sh_offset = 0;
 
@@ -1035,9 +1037,10 @@ handle_elf (int fd, Elf *elf, const char *prefix, const char *fname,
 	}
     }
 
-  /* Mark the section header string table as unused, we will create
-     a new one.  */
-  shdr_info[shstrndx].idx = 0;
+  /* Although we always create a new section header string table we
+     don't explicitly mark the existing one as unused.  It can still
+     be used through a symbol table section we are keeping.  If not it
+     will already be marked as unused.  */
 
   /* We need a string table for the section headers.  */
   shst = ebl_strtabinit (true);