libelf: Fixup SHF_COMPRESSED sh_addralign in elf_update if necessary.

In elf_getdata.c we have the following to compensate for possibly
bad sh_addralign values of compressed sections:

      /* Compressed data has a header, but then compressed data.
         Make sure to set the alignment of the header explicitly,
         don't trust the file alignment for the section, it is
         often wrong.  */
      if ((flags & SHF_COMPRESSED) != 0)
        {
          entsize = 1;
          align = __libelf_type_align (elf->class, ELF_T_CHDR);
        }

Which makes sure the d_data alignment is correct for the Chdr struct
at the start of the compressed section.

But this means that if a user just reads such a compressed section
without changing it, and then tries to write it out again using
elf_update they get an error message about d_align and sh_addralign
being out of sync.

We already correct obviously incorrect sh_entsize fields.
Do the same for the sh_addralign field of a SHF_COMPRESSED section.

Signed-off-by: Mark Wielaard <mark@klomp.org>

diff --git a/libelf/ChangeLog b/libelf/ChangeLog
index 8f6d2d2db97ab0760e09a70d2e7e7af0235c5165..77044c1cd164995efbd4a160c88acd5848a43f83 100644 (file)
--- a/libelf/ChangeLog
+++ b/libelf/ChangeLog
@@ -1,3 +1,8 @@
+2020-08-19  Mark Wielaard  <mark@klomp.org>
+
+       * elf32_updatenull.c (updatenull_wrlock): Fixup the sh_addralign
+       of an SHF_COMPRESSED section if necessary.
+
 2020-06-04  Mark Wielaard  <mark@klomp.org>
 
        * elf.h: Update from glibc.

diff --git a/libelf/elf32_updatenull.c b/libelf/elf32_updatenull.c
index 5f3cdbf67bc26b5c74d351a755615014c561870a..d0d4d1eb6f33f7562fc71eb58172fa1b7f8627e2 100644 (file)
--- a/libelf/elf32_updatenull.c
+++ b/libelf/elf32_updatenull.c
@@ -267,6 +267,18 @@ __elfw2(LIBELFBITS,updatenull_wrlock) (Elf *elf, int *change_bop, size_t shnum)
              update_if_changed (shdr->sh_entsize, sh_entsize,
                                 scn->shdr_flags);
 
+             /* Likewise for the alignment of a compressed section.
+                For a SHF_COMPRESSED section set the correct
+                sh_addralign value, which must match the d_align of
+                the data (see __libelf_set_rawdata in elf_getdata.c).  */
+             if ((shdr->sh_flags & SHF_COMPRESSED) != 0)
+               {
+                 sh_align = __libelf_type_align (ELFW(ELFCLASS,LIBELFBITS),
+                                                 ELF_T_CHDR);
+                 update_if_changed (shdr->sh_addralign, sh_align,
+                                    scn->shdr_flags);
+               }
+
              if (scn->data_read == 0
                  && __libelf_set_rawdata_wrlock (scn) != 0)
                /* Something went wrong.  The error value is already set.  */