Re: [PATCH] readelf: support zstd compressed debug sections [PR 29640]

[Alan Modra <amodra@gmail.com>]

On Fri, Oct 14, 2022 at 10:39:17PM +1030, Alan Modra wrote:
> On Fri, Oct 14, 2022 at 09:58:41PM +1030, Alan Modra wrote:
> > So we have a zlib-gabi .debug_info section that increases in size with
> > zstd, so much so that it's better to leave the section uncompressed.
> > Things go horribly wrong due to leaving compress_status as
> > COMPRESS_SECTION_NONE.  The section is read again off disk using the
> > uncompressed size.  So you get the zlib section again with some
> > garbage at the end.
> > 
> > Also, if the section is to be left uncompressed, the input
> > SHF_COMPRESSED flag needs to be reset otherwise it is copied to
> > output.
> > 
> > I'm not ready to commit this, just thought I'd post the results of a
> > bit of debugging.
> 
> And if I'd run the testsuite before posting, I may have posted a
> better patch..  Using COMPRESS_SECTION_DONE led to .debug -> .zdebug
> renaming of sections, so it appears we want another compress_status
> that says the final section contents are in sec->contents.

Another compress_status isn't elegant.  I'm about to commit this:

	* bfd.c (bfd_convert_section_contents): Handle zstd.
	* compress.c (bfd_compress_section_contents): When section
	contents are uncompressed set SEC_IN_MEMORY flag,
	compress_status to COMRESS_SECTION_NONE, and clear
	SHF_COMPRESSED.  Set SEC_IN_MEMORY for compressed contents.
	(bfd_get_full_section_contents): Don't check section size
	against file size when SEC_IN_MEMORY.
	(bfd_cache_section_contents): Delete function.
	* elf32-arm.c (elf32_arm_get_synthetic_symtab): Expand
	bfd_cache_section_contents here.
	* bfd-in2.h: Regenerate.

diff --git a/bfd/bfd-in2.h b/bfd/bfd-in2.h
index 25e1806e689..534a46439fc 100644
--- a/bfd/bfd-in2.h
+++ b/bfd/bfd-in2.h
@@ -7964,9 +7964,6 @@ bfd_byte *bfd_simple_get_relocated_section_contents
 bool bfd_get_full_section_contents
    (bfd *abfd, asection *section, bfd_byte **ptr);
 
-void bfd_cache_section_contents
-   (asection *sec, void *contents);
-
 bool bfd_is_section_compressed_info
    (bfd *abfd, asection *section,
     int *compression_header_size_p,
diff --git a/bfd/bfd.c b/bfd/bfd.c
index 5f2033bee7a..4fca8250082 100644
--- a/bfd/bfd.c
+++ b/bfd/bfd.c
@@ -2801,14 +2801,14 @@ bfd_convert_section_contents (bfd *ibfd, sec_ptr isec, bfd *obfd,
   if (ohdr_size == sizeof (Elf32_External_Chdr))
     {
       Elf32_External_Chdr *echdr = (Elf32_External_Chdr *) contents;
-      bfd_put_32 (obfd, ELFCOMPRESS_ZLIB, &echdr->ch_type);
+      bfd_put_32 (obfd, chdr.ch_type, &echdr->ch_type);
       bfd_put_32 (obfd, chdr.ch_size, &echdr->ch_size);
       bfd_put_32 (obfd, chdr.ch_addralign, &echdr->ch_addralign);
     }
   else
     {
       Elf64_External_Chdr *echdr = (Elf64_External_Chdr *) contents;
-      bfd_put_32 (obfd, ELFCOMPRESS_ZLIB, &echdr->ch_type);
+      bfd_put_32 (obfd, chdr.ch_type, &echdr->ch_type);
       bfd_put_32 (obfd, 0, &echdr->ch_reserved);
       bfd_put_64 (obfd, chdr.ch_size, &echdr->ch_size);
       bfd_put_64 (obfd, chdr.ch_addralign, &echdr->ch_addralign);
diff --git a/bfd/compress.c b/bfd/compress.c
index 364df14142b..9608a0a6341 100644
--- a/bfd/compress.c
+++ b/bfd/compress.c
@@ -151,7 +151,8 @@ bfd_compress_section_contents (bfd *abfd, sec_ptr sec)
 	  free (input_buffer);
 	  bfd_set_section_alignment (sec, uncompressed_alignment_pow);
 	  sec->contents = buffer;
-	  sec->compress_status = COMPRESS_SECTION_DONE;
+	  sec->flags |= SEC_IN_MEMORY;
+	  sec->compress_status = COMPRESS_SECTION_NONE;
 	  sec->size = uncompressed_size;
 	  input_buffer = buffer;
 	}
@@ -206,6 +207,7 @@ bfd_compress_section_contents (bfd *abfd, sec_ptr sec)
   if (compressed_size >= uncompressed_size)
     {
       memcpy (buffer, input_buffer, uncompressed_size);
+      elf_section_flags (sec) &= ~SHF_COMPRESSED;
       sec->compress_status = COMPRESS_SECTION_NONE;
     }
   else
@@ -216,6 +218,7 @@ bfd_compress_section_contents (bfd *abfd, sec_ptr sec)
       sec->compress_status = COMPRESS_SECTION_DONE;
     }
   sec->contents = buffer;
+  sec->flags |= SEC_IN_MEMORY;
   free (input_buffer);
   return uncompressed_size;
 }
@@ -268,6 +271,7 @@ bfd_get_full_section_contents (bfd *abfd, sec_ptr sec, bfd_byte **ptr)
 	  ufile_ptr filesize = bfd_get_file_size (abfd);
 	  if (filesize > 0
 	      && filesize < sz
+	      && (bfd_section_flags (sec) & SEC_IN_MEMORY) == 0
 	      /* PR 24753: Linker created sections can be larger than
 		 the file size, eg if they are being used to hold stubs.  */
 	      && (bfd_section_flags (sec) & SEC_LINKER_CREATED) == 0
@@ -380,29 +384,6 @@ bfd_get_full_section_contents (bfd *abfd, sec_ptr sec, bfd_byte **ptr)
     }
 }
 
-/*
-FUNCTION
-	bfd_cache_section_contents
-
-SYNOPSIS
-	void bfd_cache_section_contents
-	  (asection *sec, void *contents);
-
-DESCRIPTION
-	Stash @var(contents) so any following reads of @var(sec) do
-	not need to decompress again.
-*/
-
-void
-bfd_cache_section_contents (asection *sec, void *contents)
-{
-  if (sec->compress_status == DECOMPRESS_SECTION_ZLIB
-      || sec->compress_status == DECOMPRESS_SECTION_ZSTD)
-    sec->compress_status = COMPRESS_SECTION_DONE;
-  sec->contents = contents;
-  sec->flags |= SEC_IN_MEMORY;
-}
-
 /*
 FUNCTION
 	bfd_is_section_compressed_info
diff --git a/bfd/elf32-arm.c b/bfd/elf32-arm.c
index 0852b38ae4c..ec18a8ab362 100644
--- a/bfd/elf32-arm.c
+++ b/bfd/elf32-arm.c
@@ -20020,9 +20020,11 @@ elf32_arm_get_synthetic_symtab (bfd *abfd,
   data = plt->contents;
   if (data == NULL)
     {
-      if (!bfd_get_full_section_contents (abfd, (asection *) plt, &data) || data == NULL)
+      if (!bfd_get_full_section_contents (abfd, plt, &data)
+	  || data == NULL)
 	return -1;
-      bfd_cache_section_contents ((asection *) plt, data);
+      plt->contents = data;
+      plt->flags |= SEC_IN_MEMORY;
     }
 
   count = relplt->size / hdr->sh_entsize;


-- 
Alan Modra
Australia Development Lab, IBM