Re: [PATCH] elfcompress: Don't rewrite file if no section data needs to be updated.

[Mark Wielaard <mark at klomp dot org>]

Hi Igor,

On Sun, Jul 22, 2018 at 10:26:19AM +0200, Igor Gnatenko wrote:
> > With --force the file will still always be updated/rewritten even if
> > no section data needs to be (de)compressed.
> 
> I have doubts about behavior in last sentence. In --help, it's written
> "Force compression of section even if it would become larger". So usually
> people would use this option just to make sure that it would really
> compress/decompress sections.

It is only used for compression. You can use --force to compress
sections even if that makes the ELF section data larger. This is
arguably useless, so only really used for testing purposes.

> However, if nothing needs
> compression/decompression - why to rewrite file?

Indeed. That is why I wrote this patch. Because it might still change
the file. It might remove some padding between sections, correct
alignment of some data in the file, change permission/ownership, etc.

In general you wouldn't want that if not necessary for (de)compression
of some ELF section data.

Again that is arguably useless, so only really used for testing purposes
to see that elfcompress works correctly.

> Probably we should create
> new option --force-on-noop or -f -f or something like that?

If possible I would like only one option for "do something useless even
if not necessary".

The updated attached patch does update the help text:

  -f, --force          Force compression of section even if it would
                       become larger or update/rewrite the file even if
                       no section would be (de)compressed
Cheers,

Mark

>From d906260e480c4566da9173b50ee1f4fc229f4102 Mon Sep 17 00:00:00 2001
From: Mark Wielaard <mark@klomp.org>
Date: Sat, 21 Jul 2018 17:07:12 +0200
Subject: [PATCH] elfcompress: Don't rewrite file if no section data needs to
 be updated.

If the input and output file are the same and no section needs to
be updated we really don't need to rewrite the file.

Check whether any matching section is already compressed or decompressed.
Skip the section if it doesn't need to be changed. If no section data
needs updating end with success without rewriting/updating file.

With --force the file will still always be updated/rewritten even if
no section data needs to be (de)compressed.

Signed-off-by: Mark Wielaard <mark@klomp.org>
---
 src/ChangeLog     |  7 +++++++
 src/elfcompress.c | 45 ++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/src/ChangeLog b/src/ChangeLog
index 0e9ab301..39214c39 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,10 @@
+2018-07-21  Mark Wielaard  <mark@klomp.org>
+
+	* elfcompress.c (get_sections): New function.
+	(process_file): Check whether section needs to change. Don't rewrite
+	file if no section data needs changing.
+	(main): Update 'force' help text.
+
 2018-07-21  Mark Wielaard  <mark@klomp.org>
 
 	* elfcompress.c (process_file): Swap fchmod and fchown calls.
diff --git a/src/elfcompress.c b/src/elfcompress.c
index 1a0f9845..6ba6af41 100644
--- a/src/elfcompress.c
+++ b/src/elfcompress.c
@@ -1,5 +1,5 @@
 /* Compress or decompress an ELF file.
-   Copyright (C) 2015, 2016 Red Hat, Inc.
+   Copyright (C) 2015, 2016, 2018 Red Hat, Inc.
    This file is part of elfutils.
 
    This file is free software; you can redistribute it and/or modify
@@ -286,6 +286,15 @@ process_file (const char *fname)
     return (sections[ndx / WORD_BITS] & (1U << (ndx % WORD_BITS))) != 0;
   }
 
+  /* How many sections are we going to change?  */
+  size_t get_sections (void)
+  {
+    size_t s = 0;
+    for (size_t i = 0; i < shnum / WORD_BITS + 1; i++)
+      s += __builtin_popcount (sections[i]);
+    return s;
+  }
+
   int cleanup (int res)
   {
     elf_end (elf);
@@ -422,6 +431,9 @@ process_file (const char *fname)
      names change and whether there is a symbol table that might need
      to be adjusted be if the section header name table is changed.
 
+     If nothing needs changing, and the input and output file are the
+     same, we are done.
+
      Second a collection pass that creates the Elf sections and copies
      the data.  This pass will compress/decompress section data when
      needed.  And it will collect all data needed if we'll need to
@@ -464,7 +476,26 @@ process_file (const char *fname)
 
       if (section_name_matches (sname))
 	{
-	  if (shdr->sh_type != SHT_NOBITS
+	  if (!force && type == T_DECOMPRESS
+	      && (shdr->sh_flags & SHF_COMPRESSED) == 0
+	      && strncmp (sname, ".zdebug", strlen (".zdebug")) != 0)
+	    {
+	      if (verbose > 0)
+		printf ("[%zd] %s already decompressed\n", ndx, sname);
+	    }
+	  else if (!force && type == T_COMPRESS_ZLIB
+		   && (shdr->sh_flags & SHF_COMPRESSED) != 0)
+	    {
+	      if (verbose > 0)
+		printf ("[%zd] %s already compressed\n", ndx, sname);
+	    }
+	  else if (!force && type == T_COMPRESS_GNU
+		   && strncmp (sname, ".zdebug", strlen (".zdebug")) == 0)
+	    {
+	      if (verbose > 0)
+		printf ("[%zd] %s already GNU compressed\n", ndx, sname);
+	    }
+	  else if (shdr->sh_type != SHT_NOBITS
 	      && (shdr->sh_flags & SHF_ALLOC) == 0)
 	    {
 	      set_section (ndx);
@@ -518,6 +549,14 @@ process_file (const char *fname)
 	  }
     }
 
+  if (foutput == NULL && get_sections () == 0)
+    {
+      if (verbose > 0)
+	printf ("Nothing to do.\n");
+      fnew = NULL;
+      return cleanup (0);
+    }
+
   if (adjust_names)
     {
       names = dwelf_strtab_init (true);
@@ -1279,7 +1318,7 @@ main (int argc, char **argv)
 	N_("Print a message for each section being (de)compressed"),
 	0 },
       { "force", 'f', NULL, 0,
-	N_("Force compression of section even if it would become larger"),
+	N_("Force compression of section even if it would become larger or update/rewrite the file even if no section would be (de)compressed"),
 	0 },
       { "permissive", 'p', NULL, 0,
 	N_("Relax a few rules to handle slightly broken ELF files"),
-- 
2.18.0