[RFC PATCH] Mini-OS: explicitly mark symbols to be visible for apps

[Juergen Gross <jgross@xxxxxxxx>]

Add an EXPORT_SYMBOL() macro to explicitly mark a symbol to be visible
for an app or library linked with Mini-OS. This enables to hide all
other symbols from external components, avoiding any problems with
duplicate symbol names.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
Based on top of my previous series "Mini-OS: hide mini-os internal
symbols".
This is a RFC as an alternative to the map file containing all
visible symbol names. It would add more code churn, but it has the
large benefit of declaring visibility of a symbol at the place
where it is defined, reducing the chance to miss a proper symbol
visibility definition.
This patch is including an example for the do_exit() function in
order to prove the viability of the approach.
In case this idea is approved, I'd setup a patch series replacing
the second patch of above series with this patch (minus the do_exit()
example) and the scattered EXPORT_SYMBOL() instances.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 Makefile      | 3 ++-
 include/lib.h | 6 ++++++
 kernel.c      | 1 +
 mini-os.map   | 1 -
 4 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index d4768110..5dbad8ce 100644
--- a/Makefile
+++ b/Makefile
@@ -166,7 +166,8 @@ $(OBJ_DIR)/arch/x86/minios-x86%.lds:  
arch/x86/minios-x86.lds.S
 
 $(OBJ_DIR)/$(TARGET)-kern.o: $(OBJS) arch_lib 
$(OBJ_DIR)/$(TARGET_ARCH_DIR)/minios-$(MINIOS_TARGET_ARCH).lds mini-os.map
        $(LD) -r $(LDFLAGS) $(HEAD_OBJ) $(OBJS) $(LDARCHLIB) -o $@
-       $(OBJCOPY) -w -G $(GLOBAL_PREFIX)* --keep-global-symbols=mini-os.map $@ 
$@
+       $(OBJCOPY) --dump-section .export_symbol=syms mini-os-kern.o
+       $(OBJCOPY) -w -G $(GLOBAL_PREFIX)* --keep-global-symbols=mini-os.map 
--keep-global-symbols=syms --remove-section=.export_symbol $@ $@
 
 $(OBJ_DIR)/$(TARGET): $(OBJ_DIR)/$(TARGET)-kern.o $(APP_O)
        $(LD) -r $(LDFLAGS) $(OBJ_DIR)/$(TARGET)-kern.o $(APP_O) $(LDLIBS) -o 
$@.o
diff --git a/include/lib.h b/include/lib.h
index dd68985a..24fd0186 100644
--- a/include/lib.h
+++ b/include/lib.h
@@ -64,6 +64,12 @@
 #define BUILD_BUG_ON(cond) ((void)BUILD_BUG_ON_ZERO(cond))
 #endif
 
+#define EXPORT_SYMBOL(sym)                    \
+    extern typeof(sym) sym;                   \
+    asm(".section \".export_symbol\",\"a\"\n" \
+        "\t.ascii \""#sym"\n\"\n"             \
+        ".previous")
+
 #ifdef HAVE_LIBC
 #include <sys/queue.h>
 #include <sys/stat.h>
diff --git a/kernel.c b/kernel.c
index 1f97d8dd..29eed535 100644
--- a/kernel.c
+++ b/kernel.c
@@ -199,3 +199,4 @@ void do_exit(void)
         HYPERVISOR_sched_op(SCHEDOP_shutdown, &sched_shutdown);
     }
 }
+EXPORT_SYMBOL(do_exit);
diff --git a/mini-os.map b/mini-os.map
index 58a3a0ee..90f02edf 100644
--- a/mini-os.map
+++ b/mini-os.map
@@ -69,7 +69,6 @@ close
 closedir
 closelog
 connect
-do_exit
 dup
 dup2
 err
-- 
2.35.3