[Xen-devel] [PATCH] build: provide option to disambiguate symbol names

[Jan Beulich <jbeulich@xxxxxxxx>]

The .file assembler directives generated by the compiler do not include
any path components (gcc) or just the ones specified on the command line
(clang, at least version 5), and hence multiple identically named source
files (in different directories) may produce identically named static
symbols (in their kallsyms representation). The binary diffing algorithm
used by xen-livepatch, however, depends on having unique symbols.

Provide a Kconfig option to control the (build) behavior, and if enabled
use objcopy to prepend the (relative to the xen/ subdirectory) path to
the compiler invoked STT_FILE symbols.

Conditionalize explicit .file directive insertion in C files where it
exists just to disambiguate names in a less generic manner; note that
at the same time the redundant emission of STT_FILE symbols gets
suppressed for clang. Assembler files as well as multiply compiled C
ones using __OBJECT_FILE__ are left alone for the time being.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Kconfig change taken from "[PATCH v3 5/7] x86/livepatch: Fail the build
if duplicate symbols exist". When re-basing onto that other patch I
think we will also want to drop that other patch'es adjustment to
allrandom.config again.

The clang behavior may require further tweaking if different versions
behave differently. Alternatively we could pass two --redefine-sym
arguments to objcopy.

--- a/xen/Rules.mk
+++ b/xen/Rules.mk
@@ -194,12 +194,24 @@ FORCE:
 
 .PHONY: clean
 clean:: $(addprefix _clean_, $(subdir-all))
-       rm -f *.o *~ core $(DEPS_RM)
+       rm -f *.o .*.o.tmp *~ core $(DEPS_RM)
 _clean_%/: FORCE
        $(MAKE) -f $(BASEDIR)/Rules.mk -C $* clean
 
+SRCPATH := $(patsubst $(BASEDIR)/%,%,$(CURDIR))
+
 %.o: %.c Makefile
+ifeq ($(CONFIG_ENFORCE_UNIQUE_SYMBOLS),y)
+       $(CC) $(CFLAGS) -c $< -o $(@D)/.$(@F).tmp
+ifeq ($(clang),y)
+       $(OBJCOPY) --redefine-sym $<=$(SRCPATH)/$< $(@D)/.$(@F).tmp $@
+else
+       $(OBJCOPY) --redefine-sym $(<F)=$(SRCPATH)/$< $(@D)/.$(@F).tmp $@
+endif
+       rm -f $(@D)/.$(@F).tmp
+else
        $(CC) $(CFLAGS) -c $< -o $@
+endif
 
 %.o: %.S Makefile
        $(CC) $(AFLAGS) -c $< -o $@
--- a/xen/arch/x86/x86_64/compat.c
+++ b/xen/arch/x86/x86_64/compat.c
@@ -2,7 +2,7 @@
  * compat.c
  */
 
-asm(".file \"" __FILE__ "\"");
+EMIT_FILE;
 
 #include <xen/hypercall.h>
 #include <compat/xen.h>
--- a/xen/arch/x86/x86_64/mm.c
+++ b/xen/arch/x86/x86_64/mm.c
@@ -16,7 +16,7 @@
  * with this program; If not, see <http://www.gnu.org/licenses/>.
  */
 
-asm(".file \"" __FILE__ "\"");
+EMIT_FILE;
 
 #include <xen/lib.h>
 #include <xen/init.h>
--- a/xen/arch/x86/x86_64/physdev.c
+++ b/xen/arch/x86/x86_64/physdev.c
@@ -2,7 +2,7 @@
  * physdev.c
  */
 
-asm(".file \"" __FILE__ "\"");
+EMIT_FILE;
 
 #include <xen/types.h>
 #include <xen/guest_access.h>
--- a/xen/arch/x86/x86_64/platform_hypercall.c
+++ b/xen/arch/x86/x86_64/platform_hypercall.c
@@ -2,7 +2,7 @@
  * platform_hypercall.c
  */
 
-asm(".file \"" __FILE__ "\"");
+EMIT_FILE;
 
 #include <xen/lib.h>
 #include <compat/platform.h>
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -338,9 +338,23 @@ config FAST_SYMBOL_LOOKUP
 
          If unsure, say Y.
 
+config ENFORCE_UNIQUE_SYMBOLS
+       bool "Enforce unique symbols"
+       default LIVEPATCH
+       ---help---
+         Multiple symbols with the same name aren't generally a problem
+         unless Live patching is to be used.
+
+         Livepatch loading involves resolving relocations against symbol
+         names, and attempting to a duplicate symbol in a livepatch will
+         result in incorrect livepatch application.
+
+         This option should be used to ensure that a build of Xen can have a
+         livepatch build and apply correctly.
+
 config SUPPRESS_DUPLICATE_SYMBOL_WARNINGS
-       bool "Suppress duplicate symbol warnings" if !LIVEPATCH
-       default y if !LIVEPATCH
+       bool "Suppress duplicate symbol warnings"
+       depends on !ENFORCE_UNIQUE_SYMBOLS
        ---help---
          Multiple symbols with the same name aren't generally a problem
          unless Live patching is to be used, so these warnings can be
--- a/xen/common/compat/domain.c
+++ b/xen/common/compat/domain.c
@@ -3,7 +3,7 @@
  *
  */
 
-asm(".file \"" __FILE__ "\"");
+EMIT_FILE;
 
 #include <xen/lib.h>
 #include <xen/sched.h>
--- a/xen/common/compat/kernel.c
+++ b/xen/common/compat/kernel.c
@@ -2,7 +2,7 @@
  * kernel.c
  */
 
-asm(".file \"" __FILE__ "\"");
+EMIT_FILE;
 
 #include <xen/init.h>
 #include <xen/lib.h>
--- a/xen/common/compat/memory.c
+++ b/xen/common/compat/memory.c
@@ -1,4 +1,4 @@
-asm(".file \"" __FILE__ "\"");
+EMIT_FILE;
 
 #include <xen/types.h>
 #include <xen/hypercall.h>
--- a/xen/common/compat/multicall.c
+++ b/xen/common/compat/multicall.c
@@ -2,7 +2,7 @@
  * multicall.c
  */
 
-asm(".file \"" __FILE__ "\"");
+EMIT_FILE;
 
 #include <xen/types.h>
 #include <xen/multicall.h>
--- a/xen/include/xen/config.h
+++ b/xen/include/xen/config.h
@@ -11,7 +11,15 @@
 
 #ifndef __ASSEMBLY__
 #include <xen/compiler.h>
+
+#if defined(CONFIG_ENFORCE_UNIQUE_SYMBOLS) || defined(__clang__)
+# define EMIT_FILE asm ( "" )
+#else
+# define EMIT_FILE asm ( ".file \"" __FILE__ "\"" )
+#endif
+
 #endif
+
 #include <asm/config.h>
 
 #define EXPORT_SYMBOL(var)

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