[PATCH 10/10] libdwfl: Document core memory and remote memory ELF shdrs reading.

[Mark Wielaard <mark@klomp.org>]

There are two places, dwfl_segment_report_module and elf_from_remote_memory
in libdwfl where we use the Ehdr e_shnum directly. Document why this is fine.
Getting the shdrs in those two places is really just a nice bonus and if there
are more than 0xff00 then it is unlikely we will get them all anyway.

Signed-off-by: Mark Wielaard <mark@klomp.org>
---
 libdwfl/ChangeLog                    | 6 ++++++
 libdwfl/dwfl_segment_report_module.c | 6 ++++++
 libdwfl/elf-from-memory.c            | 6 ++++++
 3 files changed, 18 insertions(+)

diff --git a/libdwfl/ChangeLog b/libdwfl/ChangeLog
index 15d7511..5e9b986 100644
--- a/libdwfl/ChangeLog
+++ b/libdwfl/ChangeLog
@@ -1,3 +1,9 @@
+2018-09-13  Mark Wielaard  <mark@klomp.org>
+
+	* dwfl_segment_report_module.c (dwfl_segment_report_module):
+	Document why we use e_shnum directly.
+	* elf-from-memory.c (elf_from_remote_memory): Likewise.
+
 2018-07-17  Ulf Hermann  <ulf.hermann@qt.io>
 
 	* linux-pid-attach.c: Include sys/uio.h only on linux.
diff --git a/libdwfl/dwfl_segment_report_module.c b/libdwfl/dwfl_segment_report_module.c
index 207a257..36e5c82 100644
--- a/libdwfl/dwfl_segment_report_module.c
+++ b/libdwfl/dwfl_segment_report_module.c
@@ -367,6 +367,11 @@ dwfl_segment_report_module (Dwfl *dwfl, int ndx, const char *name,
       phentsize = ehdr.e32.e_phentsize;
       if (phentsize != sizeof (Elf32_Phdr))
 	return finish ();
+      /* NOTE if the number of sections is > 0xff00 then e_shnum
+	 is zero and the actual number would come from the section
+	 zero sh_size field. We ignore this here because getting shdrs
+	 is just a nice bonus (see below in consider_phdr PT_LOAD
+	 where we trim the last segment).  */
       shdrs_end = ehdr.e32.e_shoff + ehdr.e32.e_shnum * ehdr.e32.e_shentsize;
       break;
 
@@ -380,6 +385,7 @@ dwfl_segment_report_module (Dwfl *dwfl, int ndx, const char *name,
       phentsize = ehdr.e64.e_phentsize;
       if (phentsize != sizeof (Elf64_Phdr))
 	return finish ();
+      /* See the NOTE above for shdrs_end and ehdr.e32.e_shnum.  */
       shdrs_end = ehdr.e64.e_shoff + ehdr.e64.e_shnum * ehdr.e64.e_shentsize;
       break;
 
diff --git a/libdwfl/elf-from-memory.c b/libdwfl/elf-from-memory.c
index 12a0a1b..c54c1b9 100644
--- a/libdwfl/elf-from-memory.c
+++ b/libdwfl/elf-from-memory.c
@@ -139,6 +139,11 @@ elf_from_remote_memory (GElf_Addr ehdr_vma,
       phentsize = ehdr.e32.e_phentsize;
       if (phentsize != sizeof (Elf32_Phdr) || phnum == 0)
 	goto bad_elf;
+      /* NOTE if the number of sections is > 0xff00 then e_shnum
+	 is zero and the actual number would come from the section
+	 zero sh_size field. We ignore this here because getting shdrs
+	 is just a nice bonus (see below where we trim the last phdrs
+	 PT_LOAD segment).  */
       shdrs_end = ehdr.e32.e_shoff + ehdr.e32.e_shnum * ehdr.e32.e_shentsize;
       break;
 
@@ -151,6 +156,7 @@ elf_from_remote_memory (GElf_Addr ehdr_vma,
       phentsize = ehdr.e64.e_phentsize;
       if (phentsize != sizeof (Elf64_Phdr) || phnum == 0)
 	goto bad_elf;
+      /* See the NOTE above for shdrs_end and ehdr.e32.e_shnum.  */
       shdrs_end = ehdr.e64.e_shoff + ehdr.e64.e_shnum * ehdr.e64.e_shentsize;
       break;
 
-- 
1.8.3.1