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Date: Thu, 21 Mar 2024 08:31:16 +0200
Message-Id: <86plvo2j0r.fsf@gnu.org>
From: "Eli Zaretskii (eliz AT gnu DOT org) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
To: Pali <pali AT pali DOT im>, "Charles Sandmann" <cwsdpmi AT earthlink DOT net>
Cc: dj AT delorie DOT com, sezeroz AT gmail DOT com, djgpp AT delorie DOT com
In-Reply-To: <20240320203330.clmkn2s4rr4obq4u@pali> (message from Pali on Wed,
	20 Mar 2024 21:33:30 +0100)
Subject: Re: Error handling in __djgpp_nearptr_enable()
References: <20240309111216 DOT fetp6m34nbe6u63y AT pali> <20240320203330 DOT clmkn2s4rr4obq4u AT pali>
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> Date: Wed, 20 Mar 2024 21:33:30 +0100
> From: Pali <pali AT pali DOT im>
> Cc: djgpp AT delorie DOT com
> 
> Hello, I observed strange issue that __djgpp_nearptr_enable() function
> when running in NTVDM on Windows XP, on failure let segment limit in
> some undefined/intermediate state.
> 
> Test case:
> 
>   #include <stdio.h>
>   #include <dpmi.h>
>   #include <go32.h>
>   #include <sys/nearptr.h>
>   #include <sys/exceptn.h>
> 
>   int main() {
>     unsigned long addr;
>     if (__dpmi_get_segment_base_address(_my_ds(), &addr) == 0)
>       printf("OLD BASE: 0x%lx\n", addr);
>     printf("OLD LIMIT: 0x%lx\n", __dpmi_get_segment_limit(_my_ds()));
>     printf("CALLING __djgpp_nearptr_enable()\n");
>     if (__djgpp_nearptr_enable())
>       printf("SUCCESS\n");
>     else
>       printf("FAILED\n");
>     if (__dpmi_get_segment_base_address(_my_ds(), &addr) == 0)
>       printf("NEW BASE: 0x%lx\n", addr);
>     printf("NEW LIMIT: 0x%lx\n", __dpmi_get_segment_limit(_my_ds()));
>     return 0;
>   }
> 
> It prints:
> 
>   OLD BASE: 0x29e0000
>   OLD LIMIT: 0x9ffff
>   CALLING __djgpp_nearptr_enable()
>   FAILED
>   NEW BASE: 0x29e0000
>   NEW LIMIT: 0x7d60ffff
> 
> I know that Windows NT kernel does not allow userspace to create a
> segment with access to kernel memory space (above 0x7fff0000 limit).
> So failure from the __djgpp_nearptr_enable() call in NTVDM is expected.
> But I was not expecting that DJGPP in some cases may let segment limit
> in some intermediate state.
> 
> What about following DJGPP change? I think it can improve failure
> behavior of __djgpp_nearptr_enable() when 4 GB DS limit is not allowed.
> 
> --- src/libc/pc_hw/nearptr/nearptr.c
> +++ src/libc/pc_hw/nearptr/nearptr.c
> @@ -14,7 +14,10 @@ int __djgpp_nearptr_enable(void)
>  {
>    if(!__dpmi_set_segment_limit(_my_ds(), 0xffffffffU)) {
>      if(__dpmi_get_segment_limit(_my_ds()) != 0xffffffffU)
> +    {
> +      __dpmi_set_segment_limit(_my_ds(), __djgpp_selector_limit | 0xfff);
>        return 0;		/* We set it but DPMI ignored/truncated it */
> +    }
>      __dpmi_set_segment_limit(__djgpp_ds_alias, 0xffffffffU);
>      __dpmi_set_segment_limit(_my_cs(), 0xffffffffU);
>      _crt0_startup_flags |= _CRT0_FLAG_NEARPTR;

What are the implications of this change, in plain English, in
particular for DPMI hosts other than NTVDM?  (It would be best to
actually test this with at least the popular hosts, including CWSDPMI
and perhaps also versions of Windows newer than XP that still support
DPMI in a reasonable enough manner that allows running |DJGPP
programs.)

I'm CC'ing Charles, in the hope that he is available and can comment
on this.

Btw, the DJGPP FAQ explicitly says that __djgpp_nearptr_enable fails
on Windows NT family (i.e. with NTVDM).  So strictly speaking, a
program that calls __djgpp_nearptr_enable in that case is shooting
itself in the foot...