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From: Aaron W. LaFramboise (aaronrabiddog51_at_[hidden])
Date: 2004-08-01 21:28:30

Roland wrote:
> On Sun, 01 Aug 2004 16:41:18 -0500 "Aaron W. LaFramboise" <aaronrabiddog51_at_[hidden]> wrote:
>>Just as a FYI, I now have a copy of MSVC6, and am working on this.
>>MSVC6 does, in fact, have the necessary support, but there is a bug (I
>>had noticed this before, and this was one of the reasons I wasn't able
>>to offer more information a few months ago, and I had entirely forgotten
>>about it. Oops.). Fortunately, the bug is in the runtime library, not
>>in the linker or anything else.
> Yes the bug is, that the TLS handlers must be in a contiguous area
> between the __xl_a and __xl_z symbols. I fixed this by running a small
> piece of code during the startup (in __xi_a .. __xi_z area).

I think that the problem is something else. The linker sorts everything
correctly and puts it into a contiguous section.

The problem is apparently that noone ever used the __xl_a code (until
7.1.. what does this mean?) and so never noticed its broken. The linker
merges the sections like this:

___xl_a: .long 0 ; provided by tlssup.obj in the runtime

                .CRT$XL? ; B through Y

___xl_z: .CRT$XLZ
                .long 0 ; provided by tlssup.obj also

A relocation is generated that assigns ___xl_a to the TLS callback field
of the TLS directory. The storage referred to by ___xl_z
null-terminates the list, as specified by PECOFF.

The trouble is that we actually want the callback field to point to
___xl_a + 4, not at ___xl_a itself, which is zero. The tlssup.obj that
is part of MSVC6's runtime libraries gets this wrong, and so the TLS
callback list pointed to by the TLS directory looks like this:

[null pointer][user-specified callback][null pointer]

In other words, whatever bit of the PE loader responsible for calling
the TLS callbacks hits that first null, thinks (correctly) that it is
the end of the list, and never calls any of the callbacks.

Apparently someone noticed this, and fixed it for MSVC7.1. If you try
to use the MSVC6 tlssup.obj with MSVC7.1, you'll get the same broken
behavior. (You can't do the reverse because the objects aren't
backwards compatible.)

In any case, the runtime fixup you mention appears to fix this, although
it might be doing more work that it needs to (you just need to replace
that first zero with something valid). I must admit I am slightly
concerned about modifying an PE image at runtime to make it correct, for
the same reason I am concerned with hooking in production code. It
seems a little hackish, and it seems like it might cause suprising
behavior. The alternative is to provide an implementation of tlssup.obj
that isn't broken, but this is also slightly hackish (although it does
at least produce an image that is correct with no runtime fixups needed).

I was hoping there might be some sort of way to tweak something or other
to make the real MSVC6 tlssup.obj behave correctly, but there does not
seem to be any way other than doing some sort of runtime fixup, or
flat-out replacing the whole object.

In any case, no sort of runtime fixup should be done on anything other
than MSVC6, since later versions seem to get it right. On these
versions, I think we really should be marking the callbacks const and
using bss_seg rather than data_seg. This matches the behavior of the
rest of the native TLS support, and I think is more likely to work in

Also, on a unrelated point, is there any reason to use the .CRT$XC
section directly rather than use a global class? They're really the
same thing, but the entire .CRT section is undocumented, and not very
well known. It seems unnecessary to depend upon that interface if there
is no particular gain from using it over the well-defined interface.

Aaron W. LFramboise

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