[LibOS/vDSO] Suppress relocation of vDSO symbols

Previously, relocation of vDSO symbols was not performed when vDSO
was mapped, leading to segfaults. The relocation of these symbols
is in fact not necessary, so this commit suppresses relocation by
using GOT entries with static symbols.

LibOS/shim/src/vdso/vdso.c

@@ -21,43 +21,54 @@
 
 #include <shim_types.h>
 
-int (*__vdso_shim_clock_gettime)(clockid_t clock, struct timespec *t) = NULL;
-int (*__vdso_shim_gettimeofday)(struct timeval *tv, struct timezone *tz) = NULL;
-time_t (*__vdso_shim_time)(time_t *t) = NULL;
-long (*__vdso_shim_getcpu)(unsigned *cpu, struct getcpu_cache *unused) = NULL;
+/*
+ * The symbols below need to be exported for libsysdb to inject those values,
+ * but relocation (.rela.dyn section) isn't wanted in the code generation.
+ */
+#define EXPORT_SYMBOL(name) \
+    extern __typeof__(name) __vdso_ ## name __attribute__((alias(#name)))
 
+static int (*shim_clock_gettime)(clockid_t clock, struct timespec *t) = NULL;
+static int (*shim_gettimeofday)(struct timeval *tv, struct timezone *tz) = NULL;
+static time_t (*shim_time)(time_t *t) = NULL;
+static long (*shim_getcpu)(unsigned *cpu, struct getcpu_cache *unused) = NULL;
+
+EXPORT_SYMBOL(shim_clock_gettime);
+EXPORT_SYMBOL(shim_gettimeofday);
+EXPORT_SYMBOL(shim_time);
+EXPORT_SYMBOL(shim_getcpu);
+
+#define EXPORT_WEAK_SYMBOL(name) \
+    __typeof__(__vdso_ ## name) name __attribute__((weak, alias("__vdso_" #name)))
 
 int __vdso_clock_gettime(clockid_t clock, struct timespec *t)
 {
-    if (__vdso_shim_clock_gettime)
-        return (*__vdso_shim_clock_gettime)(clock, t);
+    if (shim_clock_gettime)
+        return (*shim_clock_gettime)(clock, t);
     return -ENOSYS;
 }
-int clock_gettime(clockid_t clock, struct timespec *t)
-    __attribute__((weak, alias("__vdso_clock_gettime")));
+EXPORT_WEAK_SYMBOL(clock_gettime);
 
 int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
 {
-    if (__vdso_shim_gettimeofday)
-        return (*__vdso_shim_gettimeofday)(tv, tz);
+    if (shim_gettimeofday)
+        return (*shim_gettimeofday)(tv, tz);
     return -ENOSYS;
 }
-int gettimeofday(struct timeval *tv, struct timezone *tz)
-    __attribute__((weak, alias("__vdso_gettimeofday")));
+EXPORT_WEAK_SYMBOL(gettimeofday);
 
 time_t __vdso_time(time_t *t)
 {
-    if (__vdso_shim_time)
-        return (*__vdso_shim_time)(t);
+    if (shim_time)
+        return (*shim_time)(t);
     return -ENOSYS;
 }
-time_t time(time_t *t) __attribute__((weak, alias("__vdso_time")));
+EXPORT_WEAK_SYMBOL(time);
 
 long __vdso_getcpu(unsigned *cpu, struct getcpu_cache *unused)
 {
-    if (__vdso_shim_getcpu)
-        return (*__vdso_shim_getcpu)(cpu, unused);
+    if (shim_getcpu)
+        return (*shim_getcpu)(cpu, unused);
     return -ENOSYS;
 }
-long getcpu(unsigned *cpu, struct getcpu_cache *unused)
-    __attribute__((weak, alias("__vdso_getcpu")));
+EXPORT_WEAK_SYMBOL(getcpu);