linux emulation: Preliminary support for more auxvec's [patch]

[Marcin Cieslak]

Alexander Leidinger wrote:
> Quoting Marcin Cieslak <saper at system.pl> (from Fri, 11 Jul 2008 21:55:36 
> +0200):
> 
>> The patch is here:
>>
>> http://akson.sgh.waw.pl/~saper/FreeBSD/linux/auxvec.diff
> 

> 
> -#define    AT_COUNT    15    /* Count of defined aux entry types. */
> +#define    AT_COUNT    18    /* Count of defined aux entry types. */
> ---snip---
> 
> I would expect that count is 24, not 18. But what happens if we increase 
> it to 24, is it expected to have some valid data by some automatism 
> based upon AT_COUNT then? What's in 18-22?

Well, the count is the count - it's number of slots in the table. It is 
used for the allocation of the structure space.

As Kostik Belousov pointed, rtld uses this (incorrectly) in 
libexec/rtld-elf/rtld.c in _rtld() an array of AT_COUNT elements is 
created and only items with values (0..AT_COUNT) are included in the table.

This should be probably be changed to something in similar to
lib/libc/gen/tls.c (see _init_tls() at the very end).

Solaris sys/auxv.h comments explain the 18-22 shift (Linux collided
with PPC ABI values):

http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/sys/auxv.h

You can also here that Solaris defines quite large AT_* values, so an 
approach taken by rtld is not practical in the long term.

I think in the long term (upstream, not in the linuxolator) we should 
allow ELF stack fixup routine to dynamically allocate user stack and get 
rid of AT_COUNT completely. I guess this could be done by extending 
(struct sysentvec *) and changing the code around 457 of kern_exec.c to 
separate "copy out strings" and "separate stack base" functions.

This work-in-progress patch 
(http://people.freebsd.org/~kib/misc/rtld-origin.1.patch) also requires 
some allocation on the user stack.

--Marcin

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