com32/chain: allow service-only runs

This patch introduces small feature, allowing so called "service-only"
runs - where the chain modules does everything as instructed, but
otherwise doesn't perform actual chainloading.

This is useful for 'mbrchs', '[un]hide[all]' and 'setbpb'+'save'
options, making possible adjustments to partition/boot sector values on
any disk/partition.

See 'chain' option in doc/chain.txt for more details.

Signed-off-by: Michal Soltys <soltys@ziu.info>

4 files changed, 63 insertions, 17 deletions

diff --git a/com32/chain/chain.c b/com32/chain/chain.c
index 25d8fa75..73d107d8 100644
--- a/com32/chain/chain.c
+++ b/com32/chain/chain.c
@@ -662,9 +662,10 @@ int main(int argc, char *argv[])
     memset(&hdat, 0, sizeof(hdat));
     memset(&sdat, 0, sizeof(sdat));
     memset(&opt, 0, sizeof(opt));
-    opt.sect = true;	/* by def load sector */
-    opt.maps = true;	/* by def map sector */
-    opt.hand = true;	/* by def prepare handover */
+    opt.sect = true;	/* by def. load sector */
+    opt.maps = true;	/* by def. map sector */
+    opt.hand = true;	/* by def. prepare handover */
+    opt.chain = true;	/* by def. do chainload */
     opt.foff = opt.soff = opt.fip = opt.sip = 0x7C00;
     opt.drivename = "boot";
 #ifdef DEBUG
@@ -777,7 +778,7 @@ int main(int argc, char *argv[])
 	goto bail;
 
     /*
-     * Prepare boot-time mmap data We should to it here, as manglers could
+     * Prepare boot-time mmap data. We should to it here, as manglers could
      * potentially alter some of the data.
      */
 
@@ -806,7 +807,10 @@ int main(int argc, char *argv[])
 	wait_key();
     }
 
-    do_boot(data, ndata);
+    if (ndata && opt.chain) /* boot only if we actually chainload */
+	do_boot(data, ndata);
+    else
+	error("Service-only run completed, exiting.\n");
 bail:
     pi_del(&iter);
     /* Free allocated areas */
diff --git a/com32/chain/options.c b/com32/chain/options.c
index 2c3d0ff8..53810da7 100644
--- a/com32/chain/options.c
+++ b/com32/chain/options.c
@@ -65,10 +65,6 @@ Usage:\n\
     chain.c32 fs [options]\n\
 ", "\
 \nOptions ('no' prefix specifies default value):\n\
-    file=<file>          Load and execute <file>\n\
-    seg=<s[:o[:i]]>      Load file at <s:o>, jump to <s:i>\n\
-                         - defaults to 0:0x7C00:0x7C00\n\
-                         - ommited o/i values default to 0\n\
     sect[=<s[:o[:i]]>]   Load sector at <s:o>, jump to <s:i>\n\
                          - defaults to 0:0x7C00:0x7C00\n\
                          - ommited o/i values default to 0x7C00\n\
@@ -78,17 +74,23 @@ Usage:\n\
     nosave               Write adjusted sector back to disk\n\
     hand                 Prepare handover area\n\
     nohptr               Force ds:si and ds:bp to point to handover area\n\
-    bss=<filename>       Emulate syslinux's BSS\n\
-    bs=<filename>        Emulate syslinux's BS\n\
     noswap               Swap drive numbers, if bootdisk is not fd0/hd0\n\
-    nohide               Hide primary partitions, unhide selected partition\n\
-    nohideall            Hide *all* partitions, unhide selected partition\n\
+    nohide               Disable all hide variations (also the default)\n\
+    hide                 Hide primary partitions, unhide selected partition\n\
+    hideall              Hide *all* partitions, unhide selected partition\n\
+    unhide               Unhide primary partitions\n\
+    unhideall            Unhide *all* partitions\n\
     nombrchs             Walk *all* partitions and fix E/MBRs' chs values\n\
     nokeeppxe            Keep the PXE and UNDI stacks in memory (PXELINUX)\n\
     nowarn               Wait for a keypress to continue chainloading\n\
                          - useful to see emited warnings\n\
+    chain                Actually perform the chainloading\n\
 ", "\
 \nOptions continued ...\n\
+    file=<file>          Load and execute <file>\n\
+    seg=<s[:o[:i]]>      Load file at <s:o>, jump to <s:i>\n\
+                         - defaults to 0:0x7C00:0x7C00\n\
+                         - ommited o/i values default to 0\n\
     isolinux=<loader>    Load another version of ISOLINUX\n\
     ntldr=<loader>       Load Windows NTLDR, SETUPLDR.BIN or BOOTMGR\n\
     cmldr=<loader>       Load Recovery Console of Windows NT/2K/XP/2003\n\
@@ -100,6 +102,8 @@ Usage:\n\
     grub=<loader>        Load GRUB Legacy stage2\n\
     grubcfg=<filename>   Set alternative config filename for GRUB Legacy\n\
     grldr=<loader>       Load GRUB4DOS grldr\n\
+    bss=<filename>       Emulate syslinux's BSS\n\
+    bs=<filename>        Emulate syslinux's BS\n\
 \nPlease see doc/chain.txt for the detailed documentation.\n\
 "
     };
@@ -268,6 +272,13 @@ int parse_args(int argc, char *argv[])
 	    opt.warn = true;
 	} else if (!strcmp(argv[i], "nowarn")) {
 	    opt.warn = false;
+	} else if (!strcmp(argv[i], "chain")) {
+	    opt.chain = true;
+	} else if (!strcmp(argv[i], "nochain")) {
+	    opt.chain = false;
+	    opt.file = NULL;
+	    opt.maps = false;
+	    opt.hand = false;
 	} else if (((argv[i][0] == 'h' || argv[i][0] == 'f')
 		    && argv[i][1] == 'd')
 		   || !strncmp(argv[i], "mbr:", 4)
@@ -302,10 +313,12 @@ int parse_args(int argc, char *argv[])
 	goto bail;
     }
 
+#if 0
     if ((!opt.maps || !opt.sect) && !opt.file) {
 	error("You have to load something.\n");
 	goto bail;
     }
+#endif
 
     if (opt.filebpb && !opt.file) {
 	error("Option 'filebpb' requires a file.\n");
diff --git a/com32/chain/options.h b/com32/chain/options.h
index ee15768c..bc244f60 100644
--- a/com32/chain/options.h
+++ b/com32/chain/options.h
@@ -29,6 +29,7 @@ struct options {
     bool filebpb;
     bool mbrchs;
     bool warn;
+    bool chain;
     uint16_t keeppxe;
     struct syslinux_rm_regs regs;
 };
diff --git a/doc/chain.txt b/doc/chain.txt
index 76234f16..c5086515 100644
--- a/doc/chain.txt
+++ b/doc/chain.txt
@@ -10,6 +10,7 @@ Chain module can perform few basic tasks:
 - load and jump to a file (also loading a sector for other purposes)
 - prepare handover data to use by a file / boot sector
 - fix different options in a file / sector / partition entries
+- perform a "service-only" run
 
 It can chainload data from both GPT and DOS partitions, as well as boot the
 first sector from a raw disk.
@@ -33,6 +34,22 @@ In most basic form, syslinux loads specified boot sector (or mbr, if not
 specified) at 0:0x7c00, prepares handover area as a standard mbr would do, and
 jumps to 0:0x7c00.
 
+A "service-only" run is possible when either:
+
+- 'nochain' is in effect
+
+or
+
+- 'nofile' and 'nomaps' are in effect
+
+This is useful for invocations such as:
+
+chain.c32 hdN M setbpb save nochain
+chain.c32 hdN mbrchs nochain
+chain.c32 hdN unhideall nochain
+
+Please see respective options for more details.
+
 
 Module invocation:
 
@@ -187,7 +204,7 @@ drive we use during chainloading is not fd0 or hd0.
 In certain situations it's useful to hide partitions - for example to make sure
 DOS gets C:. 'hide' will hide hidable primary partitions, except the one we're
 booting from. Similary, 'hideall' will hide all hidable partitions, except the
-one we're booting from. Hiding is performed only on the booting drive. Options
+one we're booting from. Hiding is performed only on the selected drive. Options
 starting with 'un' will simply unhide every partition (primary ones or all).
 Writing is only performed, if the os type values actually changed.
 
@@ -196,7 +213,7 @@ Writing is only performed, if the os type values actually changed.
 
 If you want to make a drive you're booting from totally compatible with current
 BIOS, you can use this to fix all partitions' CHS numbers. Good to silence e.g.
-FreeDOS complainig about 'logical CHS differs from physcial' of sfdisk about
+FreeDOS complainig about 'logical CHS differs from physical' of sfdisk about
 'found (...) expected (...).  Functionally seems to be mostly cosmetic, as
 Microsoft world - in cases it cares about geometry - generally sticks to values
 written in bootsectors. And the rest of the world generally doesn't care about
@@ -214,6 +231,17 @@ stacks in memory (pxelinux only).
 This option will wait for a keypress right before continuing the chainloading.
 Useful to see warnings emited by the chain module.
 
+	*chain
+	nochain
+	nochain sets: nofile nomaps nohand
+
+It is possible to trigger a "service-only" run - The chain module will do
+everything requested as usual, but it will not perform the actual chainloading.
+'nochain' option disables handover, file loading and sector mapping, as these
+are pointless in such scenario (although file might be reenabled in some future
+version, if writing to actual files becomes possible). Mainly useful for
+options 'mbrchs', '[un]hide[all]' and setbpb.
+
 	isolinux=<file>
 	sets: file=<file> nohand nosect isolinux
 
@@ -279,7 +307,7 @@ Chainloads GRUB4DOS grldr, performing additional corrections on the file
 in memory.
 
 	bss=<file>
-	sets: bss=<file> nomaps setbpb bss
+	sets: file=<file> nomaps setbpb bss
 
 This emulates syslinux's native BSS option. This loads both the file and the
 sector, adjusts BPB values in the loaded sector, then copies all possible BPB
@@ -287,7 +315,7 @@ fields to the loaded file. Everything is made with reference to the selected
 disk/partition.
 
 	bs=<file>
-	sets: bs=<file> nosect filebpb
+	sets: file=<file> nosect filebpb
 
 This emulates syslinux's native BS option. This loads the file and if possible
 - adjusts its BPB values. Everything is made with reference to the selected