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[Fwd: Re: ELF bugfixes]


From: phcoder
Subject: [Fwd: Re: ELF bugfixes]
Date: Thu, 12 Mar 2009 14:22:28 +0100
User-agent: Thunderbird 2.0.0.19 (X11/20090105)


--

Regards
Vladimir 'phcoder' Serbinenko
--- Begin Message --- Subject: Re: ELF bugfixes Date: Thu, 12 Mar 2009 14:21:45 +0100 User-agent: Thunderbird 2.0.0.19 (X11/20090105)
David Miller wrote:
From: phcoder <address@hidden>
Date: Thu, 12 Mar 2009 12:22:15 +0100

David Miller wrote:
From: phcoder <address@hidden>
Date: Thu, 12 Mar 2009 09:23:34 +0100

Index: include/grub/elf.h
===================================================================
--- include/grub/elf.h  (revision 2036)
+++ include/grub/elf.h  (working copy)
@@ -77,7 +77,7 @@
   Elf32_Half   e_shentsize;            /* Section header table entry size */
   Elf32_Half   e_shnum;                /* Section header table entry count */
   Elf32_Half   e_shstrndx;             /* Section header string table index */
-} Elf32_Ehdr;
+} __attribute__ ((packed)) Elf32_Ehdr;
There is no reason why you should need the packed attribute here.
I can't think of any cpu where this could even remotely be necessary.
And if it's not necessary, all it does it emit terribly suboptimal
code on RISC cpus.
If it doesn't make a difference in the structure why does the compiler generate 
different code?

Because packed also means that the alignment of the structure
can't be assumed to be naturally word aligned.  So the compiler
has to use byte loads and stores to load a word, for example,
on RISC machines.

This is because the packed attribute influences how the structure is
embedded into other structures.

This attribute controls two different things, not just local packing.

I knew it but normally when you parse files normally offsets aren't guaranteed to be aligned. But now it seems that elf parser is written in a way to guarantee at least some alignments. Then this part of patch probably is to be dropped or changed to proper aligned attribute

--

Regards
Vladimir 'phcoder' Serbinenko

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