[PATCH RFC] symbols: check table sizes don't change between linking passes 2 and 3

[Jan Beulich <jbeulich@xxxxxxxx>]

While sizes (and possibly positions) of the symbol table related symbols
(and as a result other ones) are expected to change from linking pass 1
to pass 2, no such change should happen anymore from pass 2 to pass 3, or
else the internally recorded symbol table wouldn't represent the ELF or
PE/COFF ones.

For comparing to be actually useful, i.e. most notably also covering the
last of the arrays emitted, symbol sizes need establishing. Make use of
the xen/linkage.h machinery to achieve that.

Suggested-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Likely other ports also want to add such checking. Really it would be nice
if we could properly abstract out shared parts of the linking steps.

RFC: Aiui this can't go in as is right now, or we would break building Xen
     with lld. Further changes are first needed to help the situation
     there.

RFC: I'm all ears towards improvement suggestions in the temporary helper
     file handling of compare-symbol-tables. Or ideally some approach to
     avoid the use of a helper file altogether.

--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -146,6 +146,7 @@ $(TARGET)-syms: $(objtree)/prelink.o $(o
                | $(objtree)/tools/symbols $(all_symbols) --sysv --sort 
$(syms-warn-dup-y) \
                > $(dot-target).1.S
        $(MAKE) $(build)=$(@D) $(dot-target).1.o
+       $(call compare-symbol-tables, $(dot-target).0.o, $(dot-target).1.o)
        $(LD) $(XEN_LDFLAGS) -T $(obj)/xen.lds $< $(build_id_linker) \
            $(orphan-handling-y) $(dot-target).1.o -o $@
        $(NM) -pa --format=sysv $@ \
@@ -226,6 +227,8 @@ endif
                | $(objtree)/tools/symbols $(all_symbols) --sysv --sort \
                > $(dot-target).1s.S
        $(MAKE) $(build)=$(@D) .$(@F).1r.o .$(@F).1s.o
+       $(call compare-symbol-tables, $(dot-target).0r.o, $(dot-target).1r.o)
+       $(call compare-symbol-tables, $(dot-target).0s.o, $(dot-target).1s.o)
        $(LD) $(call EFI_LDFLAGS,$(VIRT_BASE)) -T $(obj)/efi.lds $< \
              $(dot-target).1r.o $(dot-target).1s.o $(orphan-handling-y) \
              $(note_file_option) -o $@
--- a/xen/scripts/Kbuild.include
+++ b/xen/scripts/Kbuild.include
@@ -56,6 +56,19 @@ define filechk
        fi
 endef
 
+###
+# Compare the symbol tables of two object files.  As diff's -I option isn't
+# standardized, the name difference of the two object files needs abstracting
+# out.
+define compare-symbol-tables
+    ln -f $(1) $(@D)/.cst.$$$$; \
+    $(OBJDUMP) -t $(@D)/.cst.$$$$ > $(1).sym; \
+    ln -f $(1) $(@D)/.cst.$$$$; \
+    $(OBJDUMP) -t $(@D)/.cst.$$$$ > $(2).sym; \
+    rm -f $(@D)/.cst.$$$$
+    diff -u $(1).sym $(2).sym
+endef
+
 # as-insn: Check whether assembler supports an instruction.
 # Usage: cflags-y += $(call as-insn,CC FLAGS,"insn",option-yes,option-no)
 as-insn = $(if $(shell echo 'void _(void) { asm volatile ( $(2) ); }' \
--- a/xen/tools/symbols.c
+++ b/xen/tools/symbols.c
@@ -253,17 +253,26 @@ static void read_map(FILE *in)
        }
 }
 
-static void output_label(char *label)
+static void output_label(const char *label, bool keep)
 {
-       if (symbol_prefix_char)
-               printf(".globl %c%s\n", symbol_prefix_char, label);
-       else
-               printf(".globl %s\n", label);
-       printf("\tALGN\n");
-       if (symbol_prefix_char)
-               printf("%c%s:\n", symbol_prefix_char, label);
-       else
-               printf("%s:\n", label);
+       static bool pending;
+
+       if (pending && !keep) {
+               printf("END(CURRENT)\n");
+               printf("#undef CURRENT\n\n");
+       }
+
+       pending = label;
+       if (!label)
+               return;
+
+       if (symbol_prefix_char) {
+               printf("DATA(%c%s, ALGN)\n", symbol_prefix_char, label);
+               printf("#define CURRENT %c%s\n", symbol_prefix_char, label);
+       } else {
+               printf("DATA(%s, ALGN)\n", label);
+               printf("#define CURRENT %s\n", label);
+       }
 }
 
 /* uncompress a compressed symbol. When this function is called, the best table
@@ -331,22 +340,22 @@ static void write_src(void)
 
                return;
        }
-       printf("#include <xen/config.h>\n");
+       printf("#include <xen/linkage.h>\n");
        printf("#if BITS_PER_LONG == 64 && !defined(SYMBOLS_ORIGIN)\n");
        printf("#define PTR .quad\n");
-       printf("#define ALGN .balign 8\n");
+       printf("#define ALGN 8\n");
        printf("#else\n");
        printf("#define PTR .long\n");
-       printf("#define ALGN .balign 4\n");
+       printf("#define ALGN 4\n");
        printf("#endif\n");
 
        printf("\t.section .rodata, \"a\"\n");
 
        printf("#ifndef SYMBOLS_ORIGIN\n");
        printf("#define SYMBOLS_ORIGIN 0\n");
-       output_label("symbols_addresses");
+       output_label("symbols_addresses", false);
        printf("#else\n");
-       output_label("symbols_offsets");
+       output_label("symbols_offsets", true);
        printf("#endif\n");
        for (i = 0, ends = 0; i < table_cnt; i++) {
                printf("\tPTR\t%#llx - SYMBOLS_ORIGIN\n", table[i].addr);
@@ -370,17 +379,15 @@ static void write_src(void)
                printf("\tPTR\t%#llx - SYMBOLS_ORIGIN\n",
                       table[i].addr + table[i].size);
        }
-       printf("\n");
 
-       output_label("symbols_num_addrs");
+       output_label("symbols_num_addrs", false);
        printf("\t.long\t%d\n", table_cnt + ends);
-       printf("\n");
 
        /* table of offset markers, that give the offset in the compressed 
stream
         * every 256 symbols */
        markers = malloc(sizeof(*markers) * ((table_cnt + ends + 255) >> 8));
 
-       output_label("symbols_names");
+       output_label("symbols_names", false);
        for (i = 0, off = 0, ends = 0; i < table_cnt; i++) {
                if (((i + ends) & 0xFF) == 0)
                        markers[(i + ends) >> 8] = off;
@@ -404,15 +411,12 @@ static void write_src(void)
                printf("\t.byte 0\n");
                ++off;
        }
-       printf("\n");
 
-       output_label("symbols_markers");
+       output_label("symbols_markers", false);
        for (i = 0; i < ((table_cnt + ends + 255) >> 8); i++)
                printf("\t.long\t%d\n", markers[i]);
-       printf("\n");
 
-
-       output_label("symbols_token_table");
+       output_label("symbols_token_table", false);
        off = 0;
        for (i = 0; i < 256; i++) {
                best_idx[i] = off;
@@ -420,34 +424,27 @@ static void write_src(void)
                printf("\t.asciz\t\"%s\"\n", buf);
                off += strlen(buf) + 1;
        }
-       printf("\n");
 
-       output_label("symbols_token_index");
+       output_label("symbols_token_index", false);
        for (i = 0; i < 256; i++)
                printf("\t.short\t%d\n", best_idx[i]);
-       printf("\n");
 
-       if (!sort_by_name) {
-               free(markers);
-               return;
-       }
-
-       output_label("symbols_num_names");
-       printf("\t.long\t%d\n", table_cnt);
-       printf("\n");
-
-       /* Sorted by original symbol names and type. */
-       qsort(table, table_cnt, sizeof(*table), compare_name_orig);
-
-       output_label("symbols_sorted_offsets");
-       /* A fixed sized array with two entries: offset in the
-        * compressed stream (for symbol name), and offset in
-        * symbols_addresses (or symbols_offset). */
-       for (i = 0; i < table_cnt; i++) {
-               printf("\t.long %u, %u\n", table[i].stream_offset, 
table[i].addr_idx);
+       if (sort_by_name) {
+               output_label("symbols_num_names", false);
+               printf("\t.long\t%d\n", table_cnt);
+
+               /* Sorted by original symbol names and type. */
+               qsort(table, table_cnt, sizeof(*table), compare_name_orig);
+
+               /* A fixed sized array with two entries: offset in the
+                * compressed stream (for symbol name), and offset in
+                * symbols_addresses (or symbols_offset). */
+               output_label("symbols_sorted_offsets", false);
+               for (i = 0; i < table_cnt; i++)
+                       printf("\t.long %u, %u\n", table[i].stream_offset, 
table[i].addr_idx);
        }
-       printf("\n");
 
+       output_label(NULL, false);
        free(markers);
 }