[Xen-devel] [PATCH v2] 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.

Make the ENFORCE_UNIQUE_SYMBOLS Kconfig option 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. Note
that this build option is made no longer depend on LIVEPATCH, but merely
defaults to its setting now.

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.

Since we now expect there not to be any duplicates anymore, also don't
force the selection of the option to 'n' anymore in allrandom.config.
Similarly COVERAGE no longer suppresses duplicate symbol warnings if
enforcement is in effect, which in turn allows
SUPPRESS_DUPLICATE_SYMBOL_WARNINGS to simply depend on
!ENFORCE_UNIQUE_SYMBOLS.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: Re-base. Conditionalize COVERAGE's select.

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

--- a/xen/Kconfig.debug
+++ b/xen/Kconfig.debug
@@ -38,7 +38,7 @@ config FRAME_POINTER
 config COVERAGE
        bool "Code coverage support"
        depends on !LIVEPATCH
-       select SUPPRESS_DUPLICATE_SYMBOL_WARNINGS
+       select SUPPRESS_DUPLICATE_SYMBOL_WARNINGS if !ENFORCE_UNIQUE_SYMBOLS
        ---help---
          Enable code coverage support.
 
--- 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
@@ -373,8 +373,7 @@ config FAST_SYMBOL_LOOKUP
 
 config ENFORCE_UNIQUE_SYMBOLS
        bool "Enforce unique symbols"
-       default y
-       depends on LIVEPATCH
+       default LIVEPATCH
        ---help---
          Multiple symbols with the same name aren't generally a problem
          unless livepatching is to be used.
@@ -387,8 +386,8 @@ config ENFORCE_UNIQUE_SYMBOLS
          livepatch build and apply correctly.
 
 config SUPPRESS_DUPLICATE_SYMBOL_WARNINGS
-       bool "Suppress duplicate symbol warnings" if !ENFORCE_UNIQUE_SYMBOLS
-       default y if !ENFORCE_UNIQUE_SYMBOLS
+       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)
--- a/xen/tools/kconfig/allrandom.config
+++ b/xen/tools/kconfig/allrandom.config
@@ -2,4 +2,3 @@
 
 CONFIG_GCOV_FORMAT_AUTODETECT=y
 CONFIG_UBSAN=n
-CONFIG_ENFORCE_UNIQUE_SYMBOLS=n

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