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Date: Tue, 9 Jun 2020 12:11:55 +0530
From: Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
To: Matt Helsley <mhelsley@...are.com>
Cc: linux-kernel@...r.kernel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Steven Rostedt <rostedt@...dmis.org>,
Sami Tolvanen <samitolvanen@...gle.com>,
Julien Thierry <jthierry@...hat.com>
Subject: Re: [RFC][PATCH v4 27/32] objtool: mcount: Generic location and
relocation table types
On 6/3/20 1:20 AM, Matt Helsley wrote:
> Rather than building the exact ELF section data we need and
> avoiding libelf's conversion step, use more GElf types
> and then libelf's elfxx_xlatetof() functions to convert
> the mcount locations (GElf_Addr) and associated relocations.
>
> This converts sift_rel_mcount() so that it doesn't use the
> recordmcount wrapper. The next patch will move it out of the
> wrapper.
>
> Signed-off-by: Matt Helsley <mhelsley@...are.com>
> ---
> tools/objtool/recordmcount.c | 44 +++----------
> tools/objtool/recordmcount.h | 120 ++++++++++++++---------------------
> 2 files changed, 59 insertions(+), 105 deletions(-)
>
> diff --git a/tools/objtool/recordmcount.c b/tools/objtool/recordmcount.c
> index 06a8f8ddefa7..ef3c360a3db9 100644
> --- a/tools/objtool/recordmcount.c
> +++ b/tools/objtool/recordmcount.c
[...]
> -static uint_t *sift_rel_mcount(uint_t *mlocp,
> - unsigned const offbase,
> - Elf_Rel **const mrelpp,
> +static void sift_rel_mcount(GElf_Addr **mlocpp,
> + GElf_Sxword *r_offsetp,
> + void **const mrelpp,
> const struct section * const rels,
> unsigned const recsym_index,
> unsigned long const recval,
> - unsigned const reltype)
> + unsigned const reltype,
> + bool is_rela)
> {
> - uint_t *const mloc0 = mlocp;
> - Elf_Rel *mrelp = *mrelpp;
> - unsigned int rel_entsize = rels->sh.sh_entsize;
> - unsigned mcountsym = 0;
> + GElf_Rel *mrelp = *mrelpp;
> + GElf_Rela *mrelap = *mrelpp;
> + unsigned int mcount_sym_info = 0;
> struct reloc *reloc;
>
> list_for_each_entry(reloc, &rels->reloc_list, list) {
> - if (!mcountsym)
> - mcountsym = get_mcountsym(reloc);
> -
> - if (mcountsym == GELF_R_INFO(reloc->sym->idx, reloc->type) && !is_fake_mcount(reloc)) {
> - uint_t const addend =
> - _w(reloc->offset - recval + mcount_adjust);
> - mrelp->r_offset = _w(offbase
> - + ((void *)mlocp - (void *)mloc0));
> - Elf_r_info(mrelp, recsym_index, reltype);
> - if (rel_entsize == sizeof(Elf_Rela)) {
> - ((Elf_Rela *)mrelp)->r_addend = addend;
> - *mlocp++ = 0;
> - } else
> - *mlocp++ = addend;
> -
> - mrelp = (Elf_Rel *)(rel_entsize + (void *)mrelp);
> + unsigned long addend;
> +
> + if (!mcount_sym_info)
> + mcount_sym_info = get_mcount_sym_info(reloc);
> +
> + if (mcount_sym_info != GELF_R_INFO(reloc->sym->idx, reloc->type) || is_fake_mcount(reloc))
> + continue;
Hi Matt,
I was trying out the patch series on ppc64le and found that __mcount_loc
and .rela__mcount_loc section pairs do not get generated.
# readelf -S fs/proc/cmdline.o|grep mcount
#
Debugged the cause to get_mcountsym()'s return type. It returns reloc
type from GELF_R_INFO() and expects Elf64_Xword a.k.a unsigned long
to be the return type but get_mcountsym() returns unsigned int on 64-bit.
On power the _mcount is of relocation type R_PPC64_REL24 (info 0x170000000a),
using unsigned int truncates the value to 0xa and fails the above check.
Using below fix, that converts mcount_sym_info to use unsigned long, generates
the __mcount_loc section pairs.
--- a/tools/objtool/mcount.c
+++ b/tools/objtool/mcount.c
@@ -163,7 +163,7 @@ static int is_mcounted_section_name(char const *const txtname)
strcmp(".cpuidle.text", txtname) == 0;
}
-static unsigned int get_mcount_sym_info(struct reloc *reloc)
+static unsigned long get_mcount_sym_info(struct reloc *reloc)
{
struct symbol *sym = reloc->sym;
char const *symname = sym->name;
@@ -274,7 +274,7 @@ static int nop_mcount(struct section * const rels,
{
struct reloc *reloc;
struct section *txts = find_section_by_index(lf, rels->sh.sh_info);
- unsigned int mcount_sym_info = 0;
+ unsigned long mcount_sym_info = 0;
int once = 0;
list_for_each_entry(reloc, &rels->reloc_list, list) {
@@ -363,7 +363,7 @@ static void sift_rel_mcount(GElf_Addr **mlocpp,
{
GElf_Rel *mrelp = *mrelpp;
GElf_Rela *mrelap = *mrelpp;
- unsigned int mcount_sym_info = 0;
+ unsigned long mcount_sym_info = 0;
struct reloc *reloc;
list_for_each_entry(reloc, &rels->reloc_list, list) {
# readelf -S fs/proc/cmdline.o|grep mcount
[31] __mcount_loc PROGBITS 0000000000000000 00022f10
[32] .rela__mcount_loc RELA 0000000000000000 00022f20
> +
> + addend = reloc->offset - recval + mcount_adjust;
> + if (is_rela) {
> + mrelap->r_offset = *r_offsetp;
> + mrelap->r_info = GELF_R_INFO(recsym_index, reltype);
> + mrelap->r_addend = addend;
> + mrelap++;
> + **mlocpp = 0;
> + } else {
> + mrelp->r_offset = *r_offsetp;
> + mrelp->r_info = GELF_R_INFO(recsym_index, reltype);
> + mrelp++;
> + **mlocpp = addend;
> }
> + (*mlocpp)++;
> + r_offsetp += loc_size;
the offsets generated for rela__mcount_loc section are incorrect:
# readelf -rW fs/proc/meminfo.o
[...]
Relocation section '.rela__mcount_loc' at offset 0x59a48 contains 4 entries:
Offset Info Type Symbol's Value Symbol's Name + Addend
0000000000000000 0000000200000026 R_PPC64_ADDR64 0000000000000000 .text + c
00000a059c401f38 0000000200000026 R_PPC64_ADDR64 0000000000000000 .text + 64
0000000000000000 0000000200000026 R_PPC64_ADDR64 0000000000000000 .text + 7c
0000000000000000 0000000600000026 R_PPC64_ADDR64 0000000000000000 .init.text + c
changing the above line to *r_offsetp += loc_size and initializing
r_offset=0 in do_mcount() generates the correct offset:
# readelf -rW fs/proc/meminfo.o
[...]
Relocation section '.rela__mcount_loc' at offset 0x59a48 contains 4 entries:
Offset Info Type Symbol's Value Symbol's Name + Addend
0000000000000000 0000000200000026 R_PPC64_ADDR64 0000000000000000 .text + c
0000000000000008 0000000200000026 R_PPC64_ADDR64 0000000000000000 .text + 64
0000000000000010 0000000200000026 R_PPC64_ADDR64 0000000000000000 .text + 7c
0000000000000018 0000000600000026 R_PPC64_ADDR64 0000000000000000 .init.text + c
> }
> - *mrelpp = mrelp;
> - return mlocp;
> + *mrelpp = is_rela ? (void *)mrelap : (void *)mrelp;
> }
>
>
--
Kamalesh
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