[Nasm-devel] [PATCH] Optionally allowing BITS 64 in OMF

[Knut S. O. <bir...@an...>]

Hi!

Using NASM to do some boot strapping code and ran into trouble when trying to emit a few 64-bit instructions in the OMF object file doing the mode switching.  While I can see how the "error: obj output format does not support 64-bit code" message can be a useful reality check for application programmers, it prevents low-level programmers from doing what they want.  It if was just a harmless warning, it wouldn't be so bad, but it turns BITS 64 into BITS 16. The main trick to mixing 64-bit code into OMF and other 32-bit output formats is to avoid 64-bit sized fixups, which normally isn't too hard.

My proposal is to add a switch to NASM to allow emitting 64-bit code in output formats that aren't 64-bit capable: 
	--allow-64bit-code-anywhere

Kind Regards,
 bird.

Signed-off-by: Knut St. Osmundsen <bir...@an...>
---
From 4f54c40fa3f23eaf85f3b4d672900690252cca70 Mon Sep 17 00:00:00 2001
From: "Knut St. Osmundsen" <bir...@an...>
Date: Tue, 10 Nov 2015 22:00:21 +0100
Subject: [PATCH] New option --allow-64bit-code-anywhere for allowing 64-bit
 code in output formats that aren't 64-bit capable, at the user's own risk.

From: bird <bi...@an...>

---
 doc/nasmdoc.src | 10 ++++++++++
 nasm.c          | 13 +++++++++++--
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/doc/nasmdoc.src b/doc/nasmdoc.src
index e468248..51c8c56 100644
--- a/doc/nasmdoc.src
+++ b/doc/nasmdoc.src
@@ -1010,6 +1010,16 @@ underscore to all global and external variables, as C sometimes
 (but not always) likes it.
 
 
+\S{opt-64bitcode} The \i\c{--allow-64bit-code-anywhere} Option.
+
+The \c{--allow-64bit-code-anywhere} option allows using 64-bit
+instructions in a 32-bit or 16-bit output format.  This is useful
+in a few situations, such as when writing code switching from
+32-bit to 64-bit mode and linking into a 32-bit module.  However,
+it is important to be aware of the restriction the output format
+poses on you in terms of relocations.  Use with caution!
+
+
 \S{nasmenv} The \i\c{NASMENV} \i{Environment} Variable
 
 If you define an environment variable called \c{NASMENV}, the program
diff --git a/nasm.c b/nasm.c
index 8557b4d..401eb78 100644
--- a/nasm.c
+++ b/nasm.c
@@ -88,6 +88,7 @@ static int using_debug_info, opt_verbose_info;
 bool tasm_compatible_mode = false;
 int pass0, passn;
 int maxbits = 0;
+static bool allow_64bit_code_anywhere = false;
 int globalrel = 0;
 int globalbnd = 0;
 
@@ -616,9 +617,11 @@ struct textargs {
 
 #define OPT_PREFIX 0
 #define OPT_POSTFIX 1
+#define OPT_64BIT_CODE_ANYWHERE 2
 struct textargs textopts[] = {
     {"prefix", OPT_PREFIX},
     {"postfix", OPT_POSTFIX},
+    {"allow-64bit-code-anywhere", OPT_64BIT_CODE_ANYWHERE},
     {NULL, 0}
 };
 
@@ -804,7 +807,10 @@ static bool process_arg(char *p, char *q)
                  "    -h          show invocation summary and exit\n\n"
                  "--prefix,--postfix\n"
                  "  this options prepend or append the given argument to all\n"
-                 "  extern and global variables\n\n"
+                 "  extern and global variables\n"
+                 "--allow-64bit-code-anywhere\n"
+                 "  do not restrict 64-bit code to 64-bit capable output\n"
+                 "  formats (use with care, no complaining)\n\n"
                  "Warnings:\n");
             for (i = 0; i <= ERR_WARN_MAX; i++)
                 printf("    %-23s %s (default %s)\n",
@@ -971,6 +977,9 @@ set_warning:
                         }
                         break;
                     }
+                case OPT_64BIT_CODE_ANYWHERE:
+                    allow_64bit_code_anywhere = true;
+                    break;
                 default:
                     {
                         nasm_error(ERR_NONFATAL | ERR_NOFILE | ERR_USAGE,
@@ -2088,7 +2097,7 @@ static int get_bits(char *value)
                          "cannot specify 64-bit segment on processor below an x86-64");
             i = 16;
         }
-        if (i != maxbits) {
+        if (i != maxbits && !allow_64bit_code_anywhere) {
             nasm_error(ERR_NONFATAL,
                          "%s output format does not support 64-bit code",
                          ofmt->shortname);
-- 
2.4.1