create-klp-module: Properly align merged .parainstructions sections

When a patch is composed of multiple .o files which have
.parainstructions sections, loading the patch causes a panic:

  general protection fault: 0000 [#1] SMP
  Modules linked in: livepatch_4_9_88_1_20180518_1(OK+) livepatch_4_9_88_1_20180510_1(OK) ...
  CPU: 1 PID: 17257 Comm: insmod Tainted: G           O  K 4.9.0-6-amd64 #1 Debian 4.9.88-1
  Hardware name: HP ProLiant MicroServer Gen8, BIOS J06 11/02/2015
  task: ffff9ff3411a4480 task.stack: ffffac8f8271c000
  RIP: 0010:[<ffffffff8ae2e1d0>]  [<ffffffff8ae2e1d0>] apply_paravirt+0xc0/0x140
  RSP: 0018:ffffac8f8271f9a0  EFLAGS: 00010216
  RAX: 00010749ffffffff RBX: ffffffffc0940658 RCX: 0000000000000085
  RDX: 00000000bfebfbff RSI: ffffac8f8271f9a2 RDI: 0000000000000246
  RBP: ffffac8f8271f9a2 R08: 0000000000000085 R09: ffffffff8ae5acb0
  R10: 0000000000000001 R11: ffff9ff3544b4400 R12: ffffffffc0940660
  R13: 0000000000000000 R14: ffff9ff3544b49c0 R15: ffff9ff3b43f0800
  FS:  00007f04c1cea700(0000) GS:ffff9ff3ca640000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000560cfd63e460 CR3: 00000001455c6000 CR4: 0000000000160670
  Stack:
   401f0ff889486973 6172007172006b00 746c00650031312e 007265746e655f69
   74006e6f69007870 ffffac8f006e6f69 00ffac8f8271fa28 ffffffff8b13ae86
   ffffac8f8271fa68 ffffffffc09471ec ffffffff8b7da9eb 0000000affffffff
  Call Trace:
   [<ffffffff8b13ae86>] ? vsscanf+0x4c6/0x800
   [<ffffffff8b13b20e>] ? sscanf+0x4e/0x70
   [<ffffffff8ae52be5>] ? arch_klp_init_object_loaded+0x105/0x130
   [<ffffffff8b13b0be>] ? vsscanf+0x6fe/0x800
   [<ffffffff8b13b20e>] ? sscanf+0x4e/0x70
   [<ffffffff8aee29e8>] ? klp_init_object_loaded+0xf8/0x210
   [<ffffffff8aee2d85>] ? klp_register_patch+0x285/0x390
   [<ffffffffc09491fa>] ? patch_init+0x1fa/0x1000 [livepatch_4_9_88_1_20180518_1]
   [<ffffffffc0949000>] ? 0xffffffffc0949000
   [<ffffffff8ae0218e>] ? do_one_initcall+0x4e/0x180
   [<ffffffff8afc87dd>] ? __vunmap+0x6d/0xc0
   [<ffffffff8afc87dd>] ? __vunmap+0x6d/0xc0
   [<ffffffff8af7eaa1>] ? do_init_module+0x5b/0x1ed
   [<ffffffff8af025a6>] ? load_module+0x2596/0x2ab0
   [<ffffffff8aefed50>] ? __symbol_put+0x60/0x60
   [<ffffffff8af02d06>] ? SYSC_finit_module+0xc6/0xf0
   [<ffffffff8ae03b7d>] ? do_syscall_64+0x8d/0xf0
   [<ffffffff8b41244e>] ? entry_SYSCALL_64_after_swapgs+0x58/0xc6
  Code: 8d 7c 05 00 e8 62 f7 ff ff 0f b6 53 f9 48 8b 7b f0 48 89 ee e8 f2 f8 ff ff 49 39 dc 76 57 44 0f b6 43 09 41 80 f8 ff 75 84 0f 0b <48> 8b 10 48 8d 7d 08 48 83 e7 f8 48 89 55 00 89 ca 48 8b 74 10
  RIP  [<ffffffff8ae2e1d0>] apply_paravirt+0xc0/0x140
   RSP <ffffac8f8271f9a0>
  ---[ end trace 128c0fa6efe85d9e ]---

The panic is caused by a corrupt .klp.arch.vmlinux..parainstructions
section:

Relocation section [208] '.rela.klp.arch.vmlinux..parainstructions' for section [207] '.klp.arch.vmlinux..parainstructions' at offset 0x29dc78 contains 10 entries:
  Offset              Type            Value               Addend Name
  000000000000000000  X86_64_64       000000000000000000    +750 __get_user_pages
  0x0000000000000010  X86_64_64       000000000000000000    +823 __get_user_pages
  0x0000000000000020  X86_64_64       000000000000000000    +890 __get_user_pages
  0x0000000000000030  X86_64_64       000000000000000000    +941 __get_user_pages
  0x0000000000000040  X86_64_64       000000000000000000   +1631 __get_user_pages
  0x0000000000000050  X86_64_64       000000000000000000   +1671 __get_user_pages
  0x000000000000005c  X86_64_64       000000000000000000   +1245 handle_userfault
  0x000000000000006c  X86_64_64       000000000000000000   +1340 handle_userfault
  0x000000000000007c  X86_64_64       000000000000000000   +1417 handle_userfault
  0x000000000000008c  X86_64_64       000000000000000000   +1717 handle_userfault

The entries are supposed to be 16 bytes each, but notice they become
misaligned starting with the 'handle_userfault' entry.

This happens because the kernel linking process lies about the
.parainstructions section size, underreporting it by four bytes.  So
when two .parainstructions sections are merged together, it results in a
corrupted .klp.arch.vmlinux..parainstructions section.

Fix it by properly aligning the section before merging it with another
one.

Fixes #852.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>

kpatch-build/create-klp-module.c

@@ -292,6 +292,27 @@ static void create_klp_arch_sections(struct kpatch_elf *kelf, char *strings)
 		 * single .klp.arch.vmlinux..parainstructions section
 		 */
 		old_size = sec->data->d_size;
+
+		/*
+		 * Due to a quirk in how .parainstructions gets linked, the
+		 * section size doesn't encompass the last 4 bytes of the last
+		 * entry.  Align the old size properly before merging.
+		 */
+		if (!strcmp(base->name, ".parainstructions")) {
+			char *str;
+			static int align_mask = 0;
+
+			if (!align_mask) {
+				str = getenv("PARA_STRUCT_SIZE");
+				if (!str)
+					ERROR("PARA_STRUCT_SIZE not set");
+
+				align_mask = atoi(str) - 1;
+			}
+
+			old_size = (old_size + align_mask) & ~align_mask;
+		}
+
 		new_size = old_size + base->data->d_size;
 		sec->data->d_buf = realloc(sec->data->d_buf, new_size);
 		sec->data->d_size = new_size;