[Xen-devel] [livepatch-build-tools part3 3/3] create-diff-object: Strip all undefined entires of known size

[Pawel Wieczorkiewicz <wipawel@xxxxxxxxx>]

The patched ELF object file contains all sections and symbols as
resulted from the compilation. However, certain symbols may not be
copied over to the resulting object file, due to being unchanged or
not included for other reasons.
In such situation the resulting object file has the entire sections
copied along (with all their entries unchanged), while some of the
corresponding symbols are not copied along at all.
This leads to having incorrect dummy (STN_UNDEF) entries in the final
hotpatch ELF file.

The newly added function livepatch_strip_undefined_elements() detects
and removes all undefined RELA entries as well as their corresponding
PROGBITS section entries.
Since the sections may contain elements of unknown size (sh.sh_entsize
== 0), perform the strip only on sections with well defined entry
sizes.

After replacing the stripped rela list, it is assumed that the next
invocation of the kpatch_rebuild_rela_section_data() will adjust all
section header parameters according to the current state.

Signed-off-by: Pawel Wieczorkiewicz <wipawel@xxxxxxxxx>
Reviewed-by: Martin Pohlack <mpohlack@xxxxxxxxx>
Reviewed-by: Bjoern Doebel <doebel@xxxxxxxxx>
Reviewed-by: Norbert Manthey <nmanthey@xxxxxxxxx>
Reviewed-by: Andra-Irina Paraschiv <andraprs@xxxxxxxxxx>

CR: https://code.amazon.com/reviews/CR-7368662
---
 create-diff-object.c | 115 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 113 insertions(+), 2 deletions(-)

diff --git a/create-diff-object.c b/create-diff-object.c
index 96b6716..4f031ef 100644
--- a/create-diff-object.c
+++ b/create-diff-object.c
@@ -1473,6 +1473,13 @@ static inline int get_section_entry_size(const struct 
section *sec, struct kpatc
         return entry_size;
 }
 
+/* Check if RELA entry has undefined (STN_UNDEF) or unchanged/not-included 
elements. */
+static inline bool has_rela_undefined_element(const struct rela *rela)
+{
+       return (GELF_R_SYM(rela->rela.r_info) == STN_UNDEF) ||
+              (!rela->sym->include && (rela->sym->status == SAME));
+}
+
 static void kpatch_verify_patchability(struct kpatch_elf *kelf)
 {
        struct section *sec;
@@ -1520,8 +1527,7 @@ static void kpatch_verify_patchability(struct kpatch_elf 
*kelf)
                                                if (rela->offset < offset || 
rela->offset >= offset + entry_size)
                                                        continue;
 
-                                               if 
((GELF_R_SYM(rela->rela.r_info) == STN_UNDEF) ||
-                                                   (!rela->sym->include && 
(rela->sym->status == SAME))) {
+                                               if 
(has_rela_undefined_element(rela)) {
                                                        log_normal("section %s 
has an entry with a STN_UNDEF symbol: %s\n",
                                                                   sec->name, 
rela->sym->name ? rela->sym->name : "none");
                                                        errs++;
@@ -1889,6 +1895,109 @@ static void livepatch_create_patches_sections(struct 
kpatch_elf *kelf,
 
 }
 
+/*
+ * The patched ELF object file contains all sections and symbols as resulted
+ * from the compilation. However, certain symbols may not be copied over to
+ * the resulting object file, due to being unchanged or not included for other
+ * reasons.
+ * In such situation the resulting object file has the entire sections copied
+ * along (with all their entries unchanged), while some of the corresponding
+ * symbols are not copied along at all.
+ * This leads to having incorrect dummy (STN_UNDEF) entries in the final
+ * hotpatch ELF file.
+ * This functions removes all undefined entries of known size from both
+ * RELA and PROGBITS sections of the patched elf object.
+ */
+static void livepatch_strip_undefined_elements(struct kpatch_elf *kelf)
+{
+       struct section *sec;
+
+       list_for_each_entry(sec, &kelf->sections, list) {
+               struct rela *rela, *safe;
+               int src_offset = 0, dst_offset = 0;
+               int entry_size, align, aligned_size;
+               char *src, *dst;
+               LIST_HEAD(newrelas);
+
+               /* use RELA section to find all its undefined entries */
+               if (!is_rela_section(sec))
+                       continue;
+
+               /* only known, fixed-size entries can be stripped */
+               entry_size = get_section_entry_size(sec->base, kelf);
+               if (entry_size == 0)
+                       continue;
+
+               /* alloc buffer for new base section */
+               dst = malloc(sec->base->sh.sh_size);
+               if (!dst)
+                       ERROR("malloc");
+
+               /* iterate through all entries of a corresponding base section 
for this RELA section */
+               for ( src = sec->base->data->d_buf; src_offset < 
sec->base->sh.sh_size; src_offset += entry_size ) {
+                       bool found_valid = false;
+
+                       list_for_each_entry_safe(rela, safe, &sec->relas, list) 
{
+                               /* check all RELA elements looking for 
corresponding entry references */
+                               if (rela->offset < src_offset || rela->offset 
>= src_offset + entry_size)
+                                       continue;
+
+                               /* ignore all undefined (STN_UNDEF) or 
unchanged/not-included elements */
+                               if (has_rela_undefined_element(rela)) {
+                                       log_normal("Found a STN_UNDEF symbol %s 
in section %s\n",
+                                                  rela->sym->name, sec->name);
+                                       continue;
+                               }
+
+                               /*
+                                * A correct match has been found, so move it 
to a new list.
+                                * Original list will be destroyed along with 
the entire kelf
+                                * object, so the reference must be preserved.
+                                */
+                               found_valid = true;
+                               list_del(&rela->list);
+                               list_add_tail(&rela->list, &newrelas);
+
+                               rela->offset -= src_offset - dst_offset;
+                               rela->rela.r_offset = rela->offset;
+                       }
+
+                       /* there is a valid RELA entry, so copy current entry */
+                       if (found_valid) {
+                               /* copy base section group */
+                               memcpy(dst + dst_offset, src + src_offset, 
entry_size);
+                               dst_offset += entry_size;
+                       }
+               }
+
+               /* verify that entry_size is a divisor of aligned section size 
*/
+               align = sec->base->sh.sh_addralign;
+               aligned_size = ((sec->base->sh.sh_size + align - 1) / align) * 
align;
+               if (src_offset != aligned_size)
+                       ERROR("group size mismatch for section %s\n", 
sec->base->name);
+
+               if (!dst_offset) {
+                       /* no changed or global functions referenced */
+                       sec->status = sec->base->status = SAME;
+                       sec->include = sec->base->include = 0;
+                       free(dst);
+                       continue;
+               }
+
+               /* overwrite with new relas list */
+               list_replace(&newrelas, &sec->relas);
+
+               /*
+                * Update text section data buf and size.
+                *
+                * The rela section's data buf and size will be regenerated in
+                * kpatch_rebuild_rela_section_data().
+                */
+               sec->base->data->d_buf = dst;
+               sec->base->data->d_size = dst_offset;
+       }
+}
+
 static int is_null_sym(struct symbol *sym)
 {
        return !strlen(sym->name);
@@ -2083,6 +2192,8 @@ int main(int argc, char *argv[])
 
        log_debug("Process special sections\n");
        kpatch_process_special_sections(kelf_patched);
+       log_debug("Strip undefined elements of known size\n");
+       livepatch_strip_undefined_elements(kelf_patched);
        log_debug("Verify patchability\n");
        kpatch_verify_patchability(kelf_patched);
 
-- 
2.16.5




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Krausenstr. 38
10117 Berlin
Geschaeftsfuehrer: Christian Schlaeger, Ralf Herbrich
Ust-ID: DE 289 237 879
Eingetragen am Amtsgericht Charlottenburg HRB 149173 B



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