[PATCH] libelf: Fixup SHF_COMPRESSED sh_addralign in elf_update if necessary.

Mark Wielaard mark@klomp.org
Wed Aug 19 21:51:15 GMT 2020


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>
---
 libelf/ChangeLog          |  5 +++++
 libelf/elf32_updatenull.c | 12 ++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/libelf/ChangeLog b/libelf/ChangeLog
index 8f6d2d2d..77044c1c 100644
--- 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 5f3cdbf6..d0d4d1eb 100644
--- 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.  */
-- 
2.18.4



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