[PATCH 2/2] recordmcount: Handle sections with no non-weak symbols

Naveen N. Rao
2022-04-27T11:00:21Z

Kernel builds on powerpc are failing with the below error [1]:
CC kernel/kexec_file.o
Cannot find symbol for section 9: .text.unlikely.
kernel/kexec_file.o: failed

Since commit d1bcae833b32f1 ("ELF: Don't generate unused section
symbols") [2], binutils started dropping section symbols that it thought
were unused. This isn't an issue in general, but with kexec_file.c, gcc
is placing kexec_arch_apply_relocations[_add] into a separate
.text.unlikely section and the section symbol ".text.unlikely" is being
dropped. Due to this, recordmcount is unable to find a non-weak symbol
in .text.unlikely to generate a relocation record against.

Handle this by falling back to a weak symbol, similar to what objtool
does in commit 44f6a7c0755d8d ("objtool: Fix seg fault with Clang
non-section symbols"). Note that this approach can result in duplicate
addresses in the final vmlinux mcount location table. A previous commit
updated ftrace to skip such duplicate entries.

After this commit, relocation records for __mcount_loc for kexec_file.o
now include two entries with the weak functions
arch_kexec_apply_relocations() and arch_kexec_apply_relocation_add() as
the relocation bases:

...
0000000000000080 R_PPC64_ADDR64 .text+0x0000000000001d34
0000000000000088 R_PPC64_ADDR64 .text+0x0000000000001fec
0000000000000090 R_PPC64_ADDR64 arch_kexec_apply_relocations_add+0x000000000000000c
0000000000000098 R_PPC64_ADDR64 arch_kexec_apply_relocations+0x000000000000000c

Powerpc does not override these functions today, so these get converted
to correct offsets in the mcount location table in vmlinux.

If one or both of these weak functions are overridden in future, in the
final vmlinux mcount table, references to these will change over to the
non-weak variant which has its own mcount location entry. As such, there
will now be two entries for these functions, both pointing to the same
non-weak location. We don't need the non-weak locations since they will
never be executed. Ftrace skips the duplicate entries due to a previous
commit.

[1] https://github.com/linuxppc/issues/issues/388
[2] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=d1bcae833b32f1

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
scripts/recordmcount.h | 86 +++++++++++++++++++++++++++++++++++-------
1 file changed, 73 insertions(+), 13 deletions(-)

diff --git a/scripts/recordmcount.h b/scripts/recordmcount.h
index 1e9baa5c4fc6ef..0c79a2d2b47493 100644
--- a/scripts/recordmcount.h
+++ b/scripts/recordmcount.h
@@ -25,6 +25,7 @@
#undef sift_rel_mcount
#undef nop_mcount
#undef find_secsym_ndx
+#undef find_sym_ndx
#undef __has_rel_mcount
#undef has_rel_mcount
#undef tot_relsize
@@ -60,6 +61,7 @@
# define sift_rel_mcount sift64_rel_mcount
# define nop_mcount nop_mcount_64
# define find_secsym_ndx find64_secsym_ndx
+# define find_sym_ndx find64_sym_ndx
# define __has_rel_mcount __has64_rel_mcount
# define has_rel_mcount has64_rel_mcount
# define tot_relsize tot64_relsize
@@ -98,6 +100,7 @@
# define sift_rel_mcount sift32_rel_mcount
# define nop_mcount nop_mcount_32
# define find_secsym_ndx find32_secsym_ndx
+# define find_sym_ndx find32_sym_ndx
# define __has_rel_mcount __has32_rel_mcount
# define has_rel_mcount has32_rel_mcount
# define tot_relsize tot32_relsize
@@ -392,6 +395,51 @@ static void get_sym_str_and_relp(Elf_Shdr const *const relhdr,
*relp = rel0;
}

+/*
+ * Find a symbol in the given section containing the instruction offset passed
+ * in r_offset, to be used in generating the relocation record for the mcount
+ * location. This is used if there were no local/global symbols in the given
+ * section to be used as the base. Weak symbols are ok, and are expected to
+ * result in duplicate mcount locations which get dropped by ftrace.
+ */
+static int find_sym_ndx(unsigned const txtndx,
+ char const *const txtname,
+ uint_t *const recvalp,
+ unsigned int *sym_index,
+ Elf_Shdr const *const symhdr,
+ Elf32_Word const *symtab,
+ Elf32_Word const *symtab_shndx,
+ unsigned const r_offset,
+ Elf_Ehdr const *const ehdr)
+{
+ Elf_Sym const *const sym0 = (Elf_Sym const *)(_w(symhdr->sh_offset)
+ + (void *)ehdr);
+ unsigned const nsym = _w(symhdr->sh_size) / _w(symhdr->sh_entsize);
+ Elf_Sym const *symp;
+ unsigned t;
+
+ for (symp = sym0, t = nsym; t; --t, ++symp) {
+ if (txtndx == get_symindex(symp, symtab, symtab_shndx)) {
+ /* function symbols on ARM have quirks, avoid them */
+ if (w2(ehdr->e_machine) == EM_ARM &&
+ ELF_ST_TYPE(symp->st_info) == STT_FUNC)
+ continue;
+
+ if (r_offset >= _w(symp->st_value) &&
+ r_offset < _w(symp->st_value) + _w(symp->st_size)) {
+ *recvalp = _w(symp->st_value);
+ *sym_index = symp - sym0;
+ return 0;
+ }
+ }
+ }
+
+ fprintf(stderr, "Cannot find symbol containing offset %u for section %u: %s.\n",
+ r_offset, txtndx, txtname);
+
+ return -1;
+}
+
/*
* Look at the relocations in order to find the calls to mcount.
* Accumulate the section offsets that are found, and their relocation info,
@@ -402,9 +450,14 @@ static uint_t *sift_rel_mcount(uint_t *mlocp,
Elf_Rel **const mrelpp,
Elf_Shdr const *const relhdr,
Elf_Ehdr const *const ehdr,
- unsigned const recsym,
- uint_t const recval,
- unsigned const reltype)
+ unsigned recsym,
+ uint_t recval,
+ unsigned const reltype,
+ unsigned int no_secsym,
+ char const *const txtname,
+ Elf_Shdr const *const shdr0,
+ Elf32_Word const *symtab,
+ Elf32_Word const *symtab_shndx)
{
uint_t *const mloc0 = mlocp;
Elf_Rel *mrelp = *mrelpp;
@@ -415,6 +468,7 @@ static uint_t *sift_rel_mcount(uint_t *mlocp,
unsigned const nrel = _w(relhdr->sh_size) / rel_entsize;
unsigned mcountsym = 0;
unsigned t;
+ uint_t addend;

get_sym_str_and_relp(relhdr, ehdr, &sym0, &str0, &relp);

@@ -424,8 +478,13 @@ static uint_t *sift_rel_mcount(uint_t *mlocp,

if (mcountsym && mcountsym == Elf_r_sym(relp) &&
!is_fake_mcount(relp)) {
- uint_t const addend =
- _w(_w(relp->r_offset) - recval + mcount_adjust);
+ if (no_secsym && find_sym_ndx(w(relhdr->sh_info),
+ txtname, &recval, &recsym,
+ &shdr0[w(relhdr->sh_link)],
+ symtab, symtab_shndx,
+ _w(relp->r_offset), ehdr))
+ return 0;
+ addend = _w(_w(relp->r_offset) - recval + mcount_adjust);
mrelp->r_offset = _w(offbase
+ ((void *)mlocp - (void *)mloc0));
Elf_r_info(mrelp, recsym, reltype);
@@ -544,8 +603,6 @@ static int find_secsym_ndx(unsigned const txtndx,
return 0;
}
}
- fprintf(stderr, "Cannot find symbol for section %u: %s.\n",
- txtndx, txtname);
return -1;
}

@@ -660,22 +717,25 @@ static int do_func(Elf_Ehdr *const ehdr, char const *const fname,
goto out; /* Nothing to be done; don't append! */
}
if (txtname && is_mcounted_section_name(txtname)) {
- unsigned int recsym;
+ unsigned int recsym = 0, no_secsym = 0;
uint_t recval = 0;

symsec_sh_link = w(relhdr->sh_link);
- result = find_secsym_ndx(w(relhdr->sh_info), txtname,
+ if (find_secsym_ndx(w(relhdr->sh_info), txtname,
&recval, &recsym,
&shdr0[symsec_sh_link],
symtab, symtab_shndx,
- ehdr);
- if (result)
- goto out;
+ ehdr))
+ no_secsym = 1;

rel_entsize = _w(relhdr->sh_entsize);
mlocp = sift_rel_mcount(mlocp,
(void *)mlocp - (void *)mloc0, &mrelp,
- relhdr, ehdr, recsym, recval, reltype);
+ relhdr, ehdr, recsym, recval, reltype,
+ no_secsym, txtname, shdr0, symtab,
+ symtab_shndx);
+ if (!mlocp)
+ goto out;
} else if (txtname && (warn_on_notrace_sect || make_nop)) {
/*
* This section is ignored by ftrace, but still
--
2.35.1