livepatch: do not ignore sections with 0 size

A side effect of ignoring such sections is that symbols belonging to
them won't be resolved, and that could make relocations belonging to
other sections that reference those symbols fail.

For example it's likely to have an empty .altinstr_replacement with
symbols pointing to it, and marking the section as ignored will
prevent the symbols from being resolved, which in turn will cause any
relocations against them to fail.

In order to solve this do not ignore sections with 0 size, only ignore
sections that don't have the SHF_ALLOC flag set.

Special case such empty sections in move_payload so they are not taken
into account in order to decide whether a livepatch can be safely
re-applied after a revert.

Fixes: 98b728a7b2 ('livepatch: Disallow applying after an revert')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Tested-by: Bjoern Doebel <doebel@amazon.de>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>

xen/common/livepatch.c

@@ -300,9 +300,6 @@ static int move_payload(struct payload *payload, struct livepatch_elf *elf)
          * and .shstrtab. For the non-relocate we allocate and copy these
          * via other means - and the .rel we can ignore as we only use it
          * once during loading.
-         *
-         * Also ignore sections with zero size. Those can be for example:
-         * data, or .bss.
          */
         if ( livepatch_elf_ignore_section(elf->sec[i].sec) )
             offset[i] = UINT_MAX;
@@ -361,8 +358,17 @@ static int move_payload(struct payload *payload, struct livepatch_elf *elf)
             else if ( elf->sec[i].sec->sh_flags & SHF_WRITE )
             {
                 buf = rw_buf;
-                rw_buf_sec = i;
-                rw_buf_cnt++;
+                if ( elf->sec[i].sec->sh_size )
+                {
+                    /*
+                     * Special handling of RW empty regions: do not account for
+                     * them in order to decide whether a patch can safely be
+                     * re-applied, but assign them a load address so symbol
+                     * resolution and relocations work.
+                     */
+                    rw_buf_sec = i;
+                    rw_buf_cnt++;
+                }
             }
             else
                 buf = ro_buf;

xen/include/xen/livepatch_elf.h

@@ -48,7 +48,7 @@ int livepatch_elf_perform_relocs(struct livepatch_elf *elf);
 
 static inline bool livepatch_elf_ignore_section(const Elf_Shdr *sec)
 {
-    return !(sec->sh_flags & SHF_ALLOC) || sec->sh_size == 0;
+    return !(sec->sh_flags & SHF_ALLOC);
 }
 #endif /* __XEN_LIVEPATCH_ELF_H__ */