Re: [RFC][PATCH v4 27/32] objtool: mcount: Generic location and relocation table types

[Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>]

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@vmware.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