[Xen-devel] [livepatch-build-tools part2 v3 6/6] create-diff-object: Do not include all .rodata sections

[Pawel Wieczorkiewicz <wipawel@xxxxxxxxx>]

Older versions of GCC did not split .rodata.str sections by function.
Because of that, the entire section was always included.
The livepatch-build-tools commit [1] fixed patch creation and kept
including all .rodata.str sections, in order to maintain existing
behavior for GCC 6.1+.
This means all .rodata.str sections are always included by default,
regardless of whether they are needed or not.

During stacked hotpatch builds it leads to unnecessary accumulation of
the .rodata.str sections as each and every consecutive hotpatch module
contains all the .rodata.str sections of previous modules.

To prevent this situation, mark the .rodata.str sections for inclusion
only if they are referenced by any of the current hotpatch symbols (or
a corresponding RELA section).

Extend patchability verification to detect all non-standard, non-rela,
non-debug and non-special sections that are not referenced by any of
the symbols or RELA sections.

Rename should_include_str_section() to is_rodata_str_section().

[1] 2af6f1aa6233 Fix patch creation with GCC 6.1+

Signed-off-by: Pawel Wieczorkiewicz <wipawel@xxxxxxxxx>
Reviewed-by: Andra-Irina Paraschiv <andraprs@xxxxxxxxxx>
Reviewed-by: Bjoern Doebel <doebel@xxxxxxxxx>
Reviewed-by: Norbert Manthey <nmanthey@xxxxxxxxx>
---
v3:
* Fixed the comment above should_include_str_section()
* Renamed should_include_str_section() to is_rodata_str_section()
v2:
* Made the commit message more precise and accurate (based on
 Ross' comments to the v1 patch)
* Kept lines limited to 80 chars
---
 create-diff-object.c | 39 ++++++++++++++++++++++++++++++++-------
 1 file changed, 32 insertions(+), 7 deletions(-)

diff --git a/create-diff-object.c b/create-diff-object.c
index 8365af0..bcccb37 100644
--- a/create-diff-object.c
+++ b/create-diff-object.c
@@ -1315,14 +1315,13 @@ static bool isnumber(const char *s)
 }
 
 /*
- * String sections are always included even if unchanged.
- * The format is either:
+ * The format of string sections is either:
  * .rodata.<func>.str1.[0-9]+ (new in GCC 6.1.0)
  * or .rodata.str1.[0-9]+ (older versions of GCC)
- * For the new format we could be smarter and only include the needed
- * strings sections.
+ * For the new format we only include the needed strings sections.
+ * For the old format all string sections are always included.
  */
-static bool should_include_str_section(const char *name)
+static bool is_rodata_str_section(const char *name)
 {
 #define GCC_5_SECTION_NAME ".rodata.str1."
 #define GCC_6_SECTION_NAME ".str1."
@@ -1350,8 +1349,7 @@ static void kpatch_include_standard_elements(struct 
kpatch_elf *kelf)
 
        list_for_each_entry(sec, &kelf->sections, list) {
                /* include these sections even if they haven't changed */
-               if (is_standard_section(sec) ||
-                   should_include_str_section(sec->name)) {
+               if (is_standard_section(sec)) {
                        sec->include = 1;
                        if (sec->secsym)
                                sec->secsym->include = 1;
@@ -1362,6 +1360,20 @@ static void kpatch_include_standard_elements(struct 
kpatch_elf *kelf)
        list_entry(kelf->symbols.next, struct symbol, list)->include = 1;
 }
 
+static void kpatch_include_standard_string_elements(struct kpatch_elf *kelf)
+{
+       struct section *sec;
+
+       list_for_each_entry(sec, &kelf->sections, list) {
+               if (is_rodata_str_section(sec->name) &&
+                   is_referenced_section(sec, kelf)) {
+                       sec->include = 1;
+                       if (sec->secsym)
+                               sec->secsym->include = 1;
+               }
+       }
+}
+
 #define inc_printf(fmt, ...) \
        log_debug("%*s" fmt, recurselevel, "", ##__VA_ARGS__);
 
@@ -1541,6 +1553,17 @@ static void kpatch_verify_patchability(struct kpatch_elf 
*kelf)
                        errs++;
                }
 
+               if (sec->include) {
+                       if (!is_standard_section(sec) && !is_rela_section(sec) 
&&
+                           !is_debug_section(sec) && !is_special_section(sec)) 
{
+                               if (!is_referenced_section(sec, kelf)) {
+                                       log_normal("section %s included, but 
not referenced\n",
+                                                  sec->name);
+                                       errs++;
+                               }
+                       }
+               }
+
                /*
                 * ensure we aren't including .data.* or .bss.*
                 * (.data.unlikely is ok b/c it only has __warned vars)
@@ -2072,6 +2095,8 @@ int main(int argc, char *argv[])
        kpatch_include_debug_sections(kelf_patched);
        log_debug("Include hook elements\n");
        kpatch_include_hook_elements(kelf_patched);
+       log_debug("Include standard string elements\n");
+       kpatch_include_standard_string_elements(kelf_patched);
        log_debug("Include new globals\n");
        new_globals_exist = kpatch_include_new_globals(kelf_patched);
        log_debug("new_globals_exist = %d\n", new_globals_exist);
-- 
2.16.5




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