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digression : MSDOS 7 and SHARE.EXE revisited (Miscellaneous)

posted by ecm Homepage E-mail, Düsseldorf, Germany, 18.12.2009, 18:19

> DOS works perfectly without the
> CPM compatibility call in vector 30 of course
That doesn't matter.

> No program exists which relies on it, or if you could find one,
> it would be so old that it would probably face major problems on anything
> newer than MSDOS version 2 :-)

The CP/M compat. interface only provides MS-DOS 1.x calls which are still supported by all later versions. However, that doesn't matter either.

> You tell me "somebody touching int 30 is not a serious threat" ? Probably
> not, but it's easier to imagine than some driver modifying the share
> hooks.
No one cares about Int30. You probably didn't even think of it!

Plus I don't understand while this should be easier to imagine than someone hooking the SHARE hooks. The SHARE hooks actually are useful to hook: either because you want to handle file sharing & locking, or maybe just because you want to monitor local file access this way. As you said about the CP/M interface, "no program exists which relies on it" so hooking it would be completely useless.

> >> You could probably decide where to search for the patch location
> >> using the CALL 5 handler instead of the Int27 handler as well.
>
> No salvation here : DOS "exec" function just fills the 'call 5' location
> in the PSP from the contents of int vectors 30/31, whatever they may be.
> If you stuff FF FF FF FF FF at 0:C0, you'll find "FF FF FF FF" at the
> destination of the call 5 of all programs you'll launch thereafter. Try it
> if you don't believe me!
A. How is this related to my quoted statement here?

B. I'm pretty sure EXEC fills the PSP field at 0006h with a pointer to 0:C0, not the pointer read from 0:C1. Therefore, even with a modified instruction/pointer at 0:C0 EXEC won't create different PSPs.

C. If you stuff FF FF FF FF at DATA:90 (or whichever SHARE hook you intend to use) your method won't work either. The difference being that SHARE actually changes your pointer legitimately (although not to FF FF FF FF).

---
l

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