cygwin-1.7, pseudo-reloc v2

Charles Wilson cygwin@cwilson.fastmail.fm
Fri Jan 23 15:49:00 GMT 2009


On Fri, 23 Jan 2009 00:23:36 -0500, "Christopher Faylor" said:
>On Fri, 23 Jan 2009 00:03:35 -0500, "Charles Wilson" wrote:
>> Are there any plans to incorporate Kai Tietz' new pseudo-reloc v2
>> runtime support into cygwin-1.7?
>> 
>> http://sourceware.org/ml/binutils/2008-11/msg00004.html
>> http://sourceware.org/ml/binutils/2008-11/msg00171.html
>> http://mingw-w64.svn.sourceforge.net/viewvc/mingw-w64/trunk/mingw-w64-crt/crt/pseudo-reloc.c?revision=518&view=markup
>
> Since this is the cygwin-developers list and not the cygwin list why
> don't you humor us all a little by explaining how and where you think it
> should be added?

Okay. FWICT, how is fairly easy: we replace
src/winsup/cygwin/lib/pseudo-reloc.c (and possibly
src/winsup/mingw/pseudo-reloc.c, with associated mods to
src/winsup/mingw/pseudo-reloc-list.c) with the version at the
sourceforge link above (which was contributed to the public domain by
Kai).

Why: From Kai's original post,
> By the current implementation there are several problems I see.
> a) Auto-imported variables do not work in all cases (e.g. structure field addresses, etc).
> b) Each relocation leads to additional code needed.
? c) For 64-bit different relocation sizes are possible.
> d) If the relocations are placed within custom read-only sections the pseudo-relocator produces an assertation by attempting to write to read-only addresses.

While we (probably) don't care about the 64 bit issues as cygwin is
still strictly a 32 bit beast, it appears as though Kai's new scheme
allows (1) auto-import to work with "complex" data types, and (2) with
relocs in read-only sections -- and the runtime component (that is,
pseudo-reloc.c) supports both current pseudo-relocs (now called "v1")
and the new style ("v2").

But this will all need testing, of course -- especially to ensure that
it doesn't break existing "v1" relocs. I just wanted to know -- since
Kai's changes have now been committed to binutils CVS, although "v1"
remains ld's default for 32 bit PE platforms -- if the new cygwin-1.7
runtime component should support the new "v2" style relocations, so that
we can play with ld's new --enable-runtime-pseudo-reloc-v2 option.

Finally, there's this:
On Mon, 3 Nov 2008 10:23:04 -0500, Christopher Faylor said:
> I would be very interested in seeing this added for all PE targets.


BTW, one part of Kai's change (now in binutils) changes this (for PE
32bit):
-  link_info.pei386_runtime_pseudo_reloc = -1;
+  link_info.pei386_runtime_pseudo_reloc = 1; /* Use by default version
1.  */
That is, we will always act as tho --enable-runtime-pseudo-reloc were
specified on the command line. This means the existing behavior ("-1" --
where we DO pseudo-relocs, but warn about it unless the user explicitly
specified --enable-runtime-pseudo-reloc) is changed to "just DO it and
be quiet" unless the user specifically disables it.

I guess it's about time we made that change, but there was no discussion
of this particular bit.

--
Chuck



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