[PATCH] elflint: Add gnuld check when a NOBITS section falls inside a segment.

[PATCH] elflint: Add gnuld check when a NOBITS section falls inside a segment.

[Mark Wielaard]

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gnuld has a really bad bug where it can place a NOBITS section inside
a PT_LOAD segment. Normally that would not work. But it also makes sure
that the contents of the file is all zeros. So in practice it is actually
a PROGBITS section with all zero data. Except that other tools will think
there is an unused gap in the ELF file after the NOBITS section.

Recognize and check this pattern in elflint when --gnu is given.

Signed-off-by: Mark Wielaard <mjw@redhat.com>
---
 src/ChangeLog |  5 +++++
 src/elflint.c | 34 ++++++++++++++++++++++++++++++++--
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/src/ChangeLog b/src/ChangeLog
index a0c32fa..5be1075 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,8 @@
+2015-08-11  Mark Wielaard  <mjw@redhat.com>
+
+	* elflint.c (check_sections): When gnuld and a NOBITS section falls
+	inside a segment make sure any ELF file contents is zero.
+
 2015-07-29  Mark Wielaard  <mjw@redhat.com>
 
 	* unstrip.c (sections_flags_match): New function.
diff --git a/src/elflint.c b/src/elflint.c
index a916886..0d5f34d 100644
--- a/src/elflint.c
+++ b/src/elflint.c
@@ -3978,9 +3978,39 @@ section [%2zu] '%s' not fully contained in segment of program header entry %d\n"
 		  {
 		    if (shdr->sh_offset < phdr->p_offset + phdr->p_filesz
 			&& !is_debuginfo)
-		      ERROR (gettext ("\
+		      {
+			if (!gnuld)
+			  ERROR (gettext ("\
 section [%2zu] '%s' has type NOBITS but is read from the file in segment of program header entry %d\n"),
-			 cnt, section_name (ebl, cnt), pcnt);
+				 cnt, section_name (ebl, cnt), pcnt);
+			else
+			  {
+			    /* This is truly horrible. GNU ld might put a
+			       NOBITS section in the middle of a PT_LOAD
+			       segment, assuming the next gap in the file
+			       actually consists of zero bits...
+			       So it really is like a PROGBITS section
+			       where the data is all zeros.  Check those
+			       zero bytes are really there.  */
+			    bool bad;
+			    Elf_Data *databits;
+			    databits = elf_getdata_rawchunk (ebl->elf,
+							     shdr->sh_offset,
+							     shdr->sh_size,
+							     ELF_T_BYTE);
+			    bad = (databits == NULL
+				   || databits->d_size != shdr->sh_size);
+			    for (size_t idx = 0;
+				 idx < databits->d_size && ! bad;
+				 idx++)
+			      bad = ((char *) databits->d_buf)[idx] != 0;
+
+			    if (bad)
+			      ERROR (gettext ("\
+section [%2zu] '%s' has type NOBITS but is read from the file in segment of program header entry %d and file contents is non-zero\n"),
+				     cnt, section_name (ebl, cnt), pcnt);
+			  }
+		      }
 		  }
 		else
 		  {
-- 
2.4.3

Re: [PATCH] elflint: Add gnuld check when a NOBITS section falls inside a segment.

[Mark Wielaard]

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On Wed, 2015-08-12 at 00:16 +0200, Mark Wielaard wrote:
> gnuld has a really bad bug where it can place a NOBITS section inside
> a PT_LOAD segment. Normally that would not work. But it also makes sure
> that the contents of the file is all zeros. So in practice it is actually
> a PROGBITS section with all zero data. Except that other tools will think
> there is an unused gap in the ELF file after the NOBITS section.
> 
> Recognize and check this pattern in elflint when --gnu is given.

Updated https://fedorahosted.org/elfutils/wiki/ElflintGNU and pushed to
master.