Subject: [patch v2] fix empty environment handling
From: "stsp (stsp2 AT yandex DOT ru) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
To: djgpp AT delorie DOT com
References: <93e55dbd-cbd4-9de1-2c29-c7bb08d872f0 AT yandex DOT ru>
Message-ID: <9a9775ad-255a-407c-de95-3b503dc2046a@yandex.ru>
Date: Sat, 5 Oct 2019 01:59:39 +0300
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Hello, here is the v2 version of the
environment fix.
Difference with v1:
just this line:

+	.db	"no environment segment$"

was replaced with: +	.db	"no envseg$"

Why, you ask? Is such a change worth a patch re-send? Well, as was 
pointed to me by DJ (in another ML), the stub must be within 2K in size.Unfortunately my msg exceeded that limit. I've shortened it to stay 
within 2K. Quite an unfortunate limitation, if you ask me.


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From 65d7caeb32f21597938f49364737789727516fdb Mon Sep 17 00:00:00 2001
From: Stas Sergeev <stsp AT users DOT sourceforge DOT net>
Date: Sun, 4 Aug 2019 01:25:38 +0300
Subject: [PATCH] fix handling of empty environment

Currently if the DOS environment is empty (can happen
when starting command.com), djgpp startup produces the
corrupted environment. This patch fixes it by checking
the loop end condition before loop action.

Another problematic case is the missing environment segment.
In this case there is no work-around, so the error msg
is printed and the program terminates.
---
 src/libc/crt0/crt1.c | 12 +++++++-----
 src/stub/stub.asm    | 16 +++++++++++++---
 2 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/src/libc/crt0/crt1.c b/src/libc/crt0/crt1.c
index f0ba9919..37c7dedb 100644
--- a/src/libc/crt0/crt1.c
+++ b/src/libc/crt0/crt1.c
@@ -128,31 +128,33 @@ char *__dos_argv0;
 static void
 setup_environment(void)
 {
-  char *dos_environ = alloca(_stubinfo->env_size), *cp;
+  char *dos_environ, *cp;
   short env_selector;
   int env_count=0;
+
+  dos_environ = alloca(_stubinfo->env_size);
   movedata(_stubinfo->psp_selector, 0x2c, ds, (int)&env_selector, 2);
   movedata(env_selector, 0, ds, (int)dos_environ, _stubinfo->env_size);
   cp = dos_environ;
-  do {
+  while (*cp) { /* repeat until two NULs */
     env_count++;
     while (*cp) cp++; /* skip to NUL */
     cp++; /* skip to next character */
-  } while (*cp); /* repeat until two NULs */
+  }
   _environ = (char **)malloc((env_count+1) * sizeof(char *));
   if (_environ == 0)
     return;
 
   cp = dos_environ;
   env_count = 0;
-  do {
+  while (*cp) { /* repeat until two NULs */
     /* putenv assumes each string is malloc'd */
     _environ[env_count] = (char *)malloc(strlen(cp)+1);
     strcpy(_environ[env_count], cp);
     env_count++;
     while (*cp) cp++; /* skip to NUL */
     cp++; /* skip to next character */
-  } while (*cp); /* repeat until two NULs */
+  }
   _environ[env_count] = 0;
 
   /*
diff --git a/src/stub/stub.asm b/src/stub/stub.asm
index 62dee9e9..76b2edd6 100644
--- a/src/stub/stub.asm
+++ b/src/stub/stub.asm
@@ -160,13 +160,21 @@ resize_again:
 ;  Scan environment for "PATH=" and the stub's full name after environment
 
 	mov	es, es:[0x2c]		; get environment segment
+	mov	di, es
+	or	di, di			; check if no env
+	jnz	@f1
+	mov	al, 111
+	mov	dx, msg_no_env
+	jmpl	error
+@f1:
 	xor	di, di			; begin search for NUL/NUL (di = 0)
 ;	mov	cx, 0xff04		; effectively `infinite' loop
 	xor 	al, al
-	.db	0xa9			; "test ax,...." -- skip 2 bytes
+	jmp	@f1
 scan_environment:
 	repne
 	scasb				; search for NUL
+@f1:
 	cmpw	es:[di], 0x4150		; "PA"
 	jne	not_path
 	scasw
@@ -182,6 +190,8 @@ not_path:
 	scasb
 	jne	scan_environment	; no, still environment
 	scasw				; adjust pointer to point to prog name
+	push	es
+	push	di
 
 ;; When we are spawned from a program which has more than 20 handles in use,
 ;; all the handles passed to us by DOS are taken (since only the first 20
@@ -199,8 +209,6 @@ not_path:
 ;-----------------------------------------------------------------------------
 ;  Get DPMI information before doing anything 386-specific
 
-	push	es
-	push	di
 	xor	cx, cx			; flag for load attempt set cx = 0
 	jz	@f2			; We always jump, shorter than jmp
 @b1:
@@ -828,6 +836,8 @@ msg_not_exe:
 	.db	": not EXE$"
 msg_not_coff:
 	.db	": not COFF (Check for viruses)$"
+msg_no_env:
+	.db	"no envseg$"
 msg_no_dpmi:
 	.db	"no DPMI - Get csdpmi*b.zip$"
 msg_no_dos_memory:
-- 
2.20.1


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