[PATCH] i386: Warn upon absolute relocations being present

o Relocations generated w.r.t absolute symbols are not processed as by
  definition, absolute symbols are not to be relocated. Explicitly warn
  user about absolutions relocations present at compile time.

o These relocations get introduced either due to linker optimizations or
  some programming oversights.

o Also create a list of symbols which have been audited to be safe and
  don't emit warnings for these.

Signed-off-by: Vivek Goyal <vgoyal@in.ibm.com>
Signed-off-by: Andi Kleen <ak@suse.de>

2 files changed, 73 insertions, 11 deletions

diff --git a/arch/i386/boot/compressed/Makefile b/arch/i386/boot/compressed/Makefile
index cc28da3a881e..a661217f33ec 100644
--- a/arch/i386/boot/compressed/Makefile
+++ b/arch/i386/boot/compressed/Makefile
@@ -20,7 +20,7 @@ $(obj)/vmlinux.bin: vmlinux FORCE
         $(call if_changed,objcopy)
 
 quiet_cmd_relocs = RELOCS  $@
-      cmd_relocs = $(obj)/relocs $< > $@
+      cmd_relocs = $(obj)/relocs $< > $@;$(obj)/relocs --abs-relocs $<
 $(obj)/vmlinux.relocs: vmlinux $(obj)/relocs FORCE
         $(call if_changed,relocs)
 
diff --git a/arch/i386/boot/compressed/relocs.c b/arch/i386/boot/compressed/relocs.c
index 0551ceb21bed..468da89153c4 100644
--- a/arch/i386/boot/compressed/relocs.c
+++ b/arch/i386/boot/compressed/relocs.c
@@ -19,6 +19,33 @@ static char *strtab[MAX_SHDRS];
 static unsigned long reloc_count, reloc_idx;
 static unsigned long *relocs;
 
+/*
+ * Following symbols have been audited. There values are constant and do
+ * not change if bzImage is loaded at a different physical address than
+ * the address for which it has been compiled. Don't warn user about
+ * absolute relocations present w.r.t these symbols.
+ */
+static const char* safe_abs_relocs[] = {
+                "__kernel_vsyscall",
+                "__kernel_rt_sigreturn",
+                "__kernel_sigreturn",
+                "SYSENTER_RETURN",
+};
+
+static int is_safe_abs_reloc(const char* sym_name)
+{
+        int i, array_size;
+
+        array_size = sizeof(safe_abs_relocs)/sizeof(char*);
+
+        for(i = 0; i < array_size; i++) {
+                if (!strcmp(sym_name, safe_abs_relocs[i]))
+                        /* Match found */
+                        return 1;
+        }
+        return 0;
+}
+
 static void die(char *fmt, ...)
 {
         va_list ap;
@@ -359,9 +386,8 @@ static void print_absolute_symbols(void)
 
 static void print_absolute_relocs(void)
 {
-        int i;
-        printf("Absolute relocations\n");
-        printf("Offset     Info     Type     Sym.Value Sym.Name\n");
+        int i, printed = 0;
+
         for(i = 0; i < ehdr.e_shnum; i++) {
                 char *sym_strtab;
                 Elf32_Sym *sh_symtab;
@@ -387,6 +413,31 @@ static void print_absolute_relocs(void)
                         if (sym->st_shndx != SHN_ABS) {
                                 continue;
                         }
+
+                        /* Absolute symbols are not relocated if bzImage is
+                         * loaded at a non-compiled address. Display a warning
+                         * to user at compile time about the absolute
+                         * relocations present.
+                         *
+                         * User need to audit the code to make sure
+                         * some symbols which should have been section
+                         * relative have not become absolute because of some
+                         * linker optimization or wrong programming usage.
+                         *
+                         * Before warning check if this absolute symbol
+                         * relocation is harmless.
+                         */
+                        if (is_safe_abs_reloc(name))
+                                continue;
+
+                        if (!printed) {
+                                printf("WARNING: Absolute relocations"
+                                        " present\n");
+                                printf("Offset     Info     Type     Sym.Value "
+                                        "Sym.Name\n");
+                                printed = 1;
+                        }
+
                         printf("%08x %08x %10s %08x  %s\n",
                                 rel->r_offset,
                                 rel->r_info,
@@ -395,7 +446,9 @@ static void print_absolute_relocs(void)
                                 name);
                 }
         }
-        printf("\n");
+
+        if (printed)
+                printf("\n");
 }
 
 static void walk_relocs(void (*visit)(Elf32_Rel *rel, Elf32_Sym *sym))
@@ -508,25 +561,31 @@ static void emit_relocs(int as_text)
 
 static void usage(void)
 {
-        die("i386_reloc [--abs | --text] vmlinux\n");
+        die("relocs [--abs-syms |--abs-relocs | --text] vmlinux\n");
 }
 
 int main(int argc, char **argv)
 {
-        int show_absolute;
+        int show_absolute_syms, show_absolute_relocs;
         int as_text;
         const char *fname;
         FILE *fp;
         int i;
 
-        show_absolute = 0;
+        show_absolute_syms = 0;
+        show_absolute_relocs = 0;
         as_text = 0;
         fname = NULL;
         for(i = 1; i < argc; i++) {
                 char *arg = argv[i];
                 if (*arg == '-') {
-                        if (strcmp(argv[1], "--abs") == 0) {
-                                show_absolute = 1;
+                        if (strcmp(argv[1], "--abs-syms") == 0) {
+                                show_absolute_syms = 1;
+                                continue;
+                        }
+
+                        if (strcmp(argv[1], "--abs-relocs") == 0) {
+                                show_absolute_relocs = 1;
                                 continue;
                         }
                         else if (strcmp(argv[1], "--text") == 0) {
@@ -553,8 +612,11 @@ int main(int argc, char **argv)
         read_strtabs(fp);
         read_symtabs(fp);
         read_relocs(fp);
-        if (show_absolute) {
+        if (show_absolute_syms) {
                 print_absolute_symbols();
+                return 0;
+        }
+        if (show_absolute_relocs) {
                 print_absolute_relocs();
                 return 0;
         }