ppc64le __ftr_fixup / __ftr_alt_97 sections causing bad final link

[joe-lawrence]

On rhel-10.2, a kpatch touching fs/userfaultfd.o results in a final linker error: ld: final link failed: bad value

Tracing through this, it seems that upstream v6.1+ 9c7bfc2dc21e ("powerpc/64s: Make POWER10 and later use pause_short in cpu_relax loops") results in way more __ftr_alt_97, __ftr_fixup, .rela__ftr_fixup sections for which kpatch-build now stumbles over:

Some debugging from the main linux tree:

Before 9c7bfc2dc21e
-------------------

$ git checkout 9c7bfc2dc21e~1

$ make ARCH=powerpc CROSS_COMPILE=powerpc64le-linux-gnu- defconfig
$ ./scripts/config --set-val CONFIG_USERFAULTFD y
$ make ARCH=powerpc CROSS_COMPILE=powerpc64le-linux-gnu- olddefconfig

$ make ARCH=powerpc CROSS_COMPILE=powerpc64le-linux-gnu- -j$(nproc) fs/userfaultfd.o
$ ppc64le-linux-gnu-readelf --sections --wide fs/userfaultfd.o | grep 'ftr_'
(nothing)


After 9c7bfc2dc21e
-------------------

$ git checkout 9c7bfc2dc21e

$ make ARCH=powerpc CROSS_COMPILE=powerpc64le-linux-gnu- defconfig
$ ./scripts/config --set-val CONFIG_USERFAULTFD y
$ make ARCH=powerpc CROSS_COMPILE=powerpc64le-linux-gnu- olddefconfig

$ make ARCH=powerpc CROSS_COMPILE=powerpc64le-linux-gnu- -j$(nproc) fs/userfaultfd.o
$ ppc64le-linux-gnu-readelf --sections --wide fs/userfaultfd.o | grep 'ftr_'
  [19] __ftr_alt_97      PROGBITS        0000000000000000 004ca0 000018 00   A  0   0  4
  [20] __ftr_fixup       PROGBITS        0000000000000000 004cb8 000120 00   A  0   0  8
  [21] .rela__ftr_fixup  RELA            0000000000000000 009b58 000240 18   I 38  20  8

$ ppc64le-linux-gnu-objdump -Dr -j __ftr_alt_97 -j__ftr_fixup fs/userfaultfd.o 

fs/userfaultfd.o:     file format elf64-powerpc


Disassembly of section __ftr_alt_97:

0000000000000000 <__ftr_alt_97>:
   0:   7c 40 00 3c     pause_short
   4:   7c 40 00 3c     pause_short
   8:   7c 40 00 3c     pause_short
   c:   7c 40 00 3c     pause_short
  10:   7c 40 00 3c     pause_short
  14:   7c 40 00 3c     pause_short

Disassembly of section __ftr_fixup:

0000000000000000 <__ftr_fixup>:
   0:   00 04 00 00     .long 0x40000
        ...
                        10: R_PPC64_REL64       .text+0x14a0
                        18: R_PPC64_REL64       .text+0x14b0
                        20: R_PPC64_REL64       __ftr_alt_97+0x20
                        28: R_PPC64_REL64       __ftr_alt_97+0x2c
  30:   00 04 00 00     .long 0x40000
        ...
                        40: R_PPC64_REL64       .text+0x1670
                        48: R_PPC64_REL64       .text+0x1680
                        50: R_PPC64_REL64       __ftr_alt_97+0x24
                        58: R_PPC64_REL64       __ftr_alt_97+0x30
  60:   00 04 00 00     .long 0x40000
        ...
                        70: R_PPC64_REL64       .text+0x17f0
                        78: R_PPC64_REL64       .text+0x1800
                        80: R_PPC64_REL64       __ftr_alt_97+0x28
                        88: R_PPC64_REL64       __ftr_alt_97+0x34
  90:   00 04 00 00     .long 0x40000
        ...
                        a0: R_PPC64_REL64       .text+0x1e00
                        a8: R_PPC64_REL64       .text+0x1e10
                        b0: R_PPC64_REL64       __ftr_alt_97+0x2c
                        b8: R_PPC64_REL64       __ftr_alt_97+0x38
  c0:   00 04 00 00     .long 0x40000
        ...
                        d0: R_PPC64_REL64       .text+0x2060
                        d8: R_PPC64_REL64       .text+0x2070
                        e0: R_PPC64_REL64       __ftr_alt_97+0x30
                        e8: R_PPC64_REL64       __ftr_alt_97+0x3c
  f0:   00 04 00 00     .long 0x40000
        ...
                        100: R_PPC64_REL64      .text+0x2350
                        108: R_PPC64_REL64      .text+0x2360
                        110: R_PPC64_REL64      __ftr_alt_97+0x34
                        118: R_PPC64_REL64      __ftr_alt_97+0x40

[joe-lawrence]

WIP: only build / load tested

diff --git a/kpatch-build/create-diff-object.c b/kpatch-build/create-diff-object.c
index 051f8e8..3d01e71 100644
--- a/kpatch-build/create-diff-object.c
+++ b/kpatch-build/create-diff-object.c
@@ -2937,7 +2937,7 @@ static void kpatch_regenerate_special_section(struct kpatch_elf *kelf,
 				rela->offset -= src_offset - dest_offset;
 				rela->rela.r_offset = rela->offset;
 
-				rela->sym->include = 1;
+				kpatch_include_symbol(rela->sym);
 
 				if (!strcmp(special->name, ".fixup"))
 					kpatch_update_ex_table_addend(kelf, special,
@@ -3079,6 +3079,20 @@ next:
 	ip_sec->data->d_size = dest_idx * ORC_IP_PTR_SIZE;
 }
 
+static bool is_ftr_alt_fixup_reloc(struct section *relasec, struct rela *rela)
+{
+	if (!relasec->base)
+		return false;
+
+	if (strncmp(relasec->base->name, "__ftr_fixup", 11) &&
+	    strncmp(relasec->base->name, "__mmu_ftr_fixup", 15) &&
+	    strncmp(relasec->base->name, "__fw_ftr_fixup", 14))
+		return false;
+
+	return rela->sym->type == STT_SECTION &&
+		!strncmp(rela->sym->name, "__ftr_alt_", 10);
+}
+
 static void kpatch_check_relocations(struct kpatch_elf *kelf)
 {
 	struct rela *rela;
@@ -3092,6 +3106,8 @@ static void kpatch_check_relocations(struct kpatch_elf *kelf)
 		list_for_each_entry(rela, &relasec->relas, list) {
 			if (!rela->sym->sec)
 				continue;
+			if (is_ftr_alt_fixup_reloc(relasec, rela))
+				continue;
 
 			sec_size = rela->sym->sec->data->d_size;
 			sec_off = (long)rela->sym->sym.st_value +