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btf_loader: Simplify fixup code by relying on BTF data more 

btf_loader relies on guessing base integral type size for enums and
integers, which is unreliable. There doesn't seem to be a need for that,
as all this information could be extracted from BTF information.

  Before:
  $ PAHOLE=~/local/pahole/build/pahole ./btfdiff ~/local/btf/libc-2.28.so.debug
  base_type__name_to_size: base_type _Float128
  class__fixup_btf_bitfields: unknown base type name "_Float128"!
  base_type__name_to_size: base_type _Float128
  class__fixup_btf_bitfields: unknown base type name "_Float128"!
  base_type__name_to_size: base_type _Float128
  class__fixup_btf_bitfields: unknown base type name "_Float128"!
  base_type__name_to_size: base_type _Float128
  class__fixup_btf_bitfields: unknown base type name "_Float128"!
  base_type__name_to_size: base_type _Float128
  class__fixup_btf_bitfields: unknown base type name "_Float128"!
  base_type__name_to_size: base_type _Float128
  class__fixup_btf_bitfields: unknown base type name "_Float128"!
  --- /tmp/btfdiff.dwarf.aV4wSL   2019-02-25 13:31:54.787923673 -0800
  +++ /tmp/btfdiff.btf.22NQmJ     2019-02-25 13:31:54.802923668 -0800
  @@ -461,9 +461,15 @@ struct La_x86_64_retval {
          uint64_t                   lrv_rdx;              /*     8     8 */
          La_x86_64_xmm              lrv_xmm0;             /*    16    16 */
          La_x86_64_xmm              lrv_xmm1;             /*    32    16 */
  -       long double                lrv_st0;              /*    48    16 */
  +       long double                lrv_st0;              /*    48     8 */
  +
  +       /* XXX 8 bytes hole, try to pack */
  +
          /* --- cacheline 1 boundary (64 bytes) --- */
  -       long double                lrv_st1;              /*    64    16 */
  +       long double                lrv_st1;              /*    64     8 */
  +
  +       /* XXX 8 bytes hole, try to pack */
  +
          La_x86_64_vector           lrv_vector0;          /*    80    64 */
          /* --- cacheline 2 boundary (128 bytes) was 16 bytes ago --- */
          La_x86_64_vector           lrv_vector1;          /*   144    64 */
  @@ -472,6 +478,7 @@ struct La_x86_64_retval {
          __int128                   lrv_bnd1;             /*   224    16 */

          /* size: 240, cachelines: 4, members: 10 */
  +       /* sum members: 224, holes: 2, sum holes: 16 */
          /* last cacheline: 48 bytes */
   };
   struct r_debug {
  @@ -2044,7 +2051,7 @@ union ieee754_float {
          } ieee_nan;                                    /*     0     4 */
   };
   union ieee854_long_double {
  -       long double                d;                  /*     0    16 */
  +       long double                d;                  /*     0     8 */
          struct {
                  unsigned int       mantissa1:32;       /*     0: 0  4 */
                  unsigned int       mantissa0:32;       /*     4: 0  4 */
  @@ -2141,7 +2148,7 @@ struct ucontext_t {
          /* last cacheline: 8 bytes */
   };
   union ieee854_float128 {
  -       _Float128                  d;                  /*     0    16 */
  +       _Float128                  d;                  /*     0     0 */
          struct {
                  unsigned int       mantissa3:32;       /*     0: 0  4 */
                  unsigned int       mantissa2:32;       /*     4: 0  4 */
  @@ -2219,7 +2226,7 @@ union printf_arg {
          long unsigned int          pa_u_long_int;      /*     0     8 */
          long long unsigned int     pa_u_long_long_int; /*     0     8 */
          double                     pa_double;          /*     0     8 */
  -       long double                pa_long_double;     /*     0    16 */
  +       long double                pa_long_double;     /*     0     8 */
          const char  *              pa_string;          /*     0     8 */
          const wchar_t  *           pa_wstring;         /*     0     8 */
          void *                     pa_pointer;         /*     0     8 */

  $ PAHOLE=~/local/pahole/build/pahole ./btfdiff ~/local/btf/libc-2.28.so.debug
  <empty output>

  Still good for kernel image:
  $ PAHOLE=~/local/pahole/build/pahole ./btfdiff ~/local/btf/vmlinux4
  <empty output>

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: bpf@vger.kernel.org
Cc: dwarves@vger.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
This commit is contained in:
Andrii Nakryiko 2019-02-25 17:19:26 -08:00 committed by Arnaldo Carvalho de Melo
parent be5173b4df
commit 8f4f280163
1 changed files with 11 additions and 48 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

59
btf_loader.c
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@ -463,64 +463,27 @@ static int class__fixup_btf_bitfields(struct tag *tag, struct cu *cu, struct btf
continue;
pos->bitfield_offset = 0;
pos->byte_size = tag__size(type, cu);
pos->bit_size = pos->byte_size * 8;
uint16_t type_bit_size;
size_t integral_bit_size;
switch (type->tag) {
case DW_TAG_enumeration_type:
type_bit_size = tag__type(type)->size;
/* Best we can do to check if this is a packed enum */
if (is_power_of_2(type_bit_size))
integral_bit_size = roundup(type_bit_size, 8);
else
integral_bit_size = sizeof(int) * 8;
break;
case DW_TAG_base_type: {
struct base_type *bt = tag__base_type(type);
char name[256];
type_bit_size = bt->bit_size;
integral_bit_size = base_type__name_to_size(bt, cu);
if (integral_bit_size == 0) {
fprintf(stderr, "%s: unknown base type name \"%s\"!\n",
__func__, base_type__name(bt, cu, name,
sizeof(name)));
}
}
break;
default:
pos->byte_size = tag__size(type, cu);
pos->bit_size = pos->byte_size * 8;
/* bitfield fixup is needed for enums and base types only */
if (type->tag != DW_TAG_base_type && type->tag != DW_TAG_enumeration_type)
continue;
}
/*
* XXX: integral_bit_size can be zero if base_type__name_to_size doesn't
* know about the base_type name, so one has to add there when
* such base_type isn't found. pahole will put zero on the
* struct output so it should be easy to spot the name when
* such unlikely thing happens.
*/
pos->byte_size = integral_bit_size / 8;
pos->bit_size = type_bit_size;
if (integral_bit_size == 0) {
pos->bit_size = 0;
/* if BTF data is incorrect and has size == 0, skip field,
* instead of crashing */
if (pos->byte_size == 0) {
continue;
}
if (pos->bitfield_size) {
/* bitfields seem to be always aligned, no matter the packing */
pos->byte_offset = pos->bit_offset / integral_bit_size * integral_bit_size / 8;
} else {
pos->byte_offset = pos->bit_offset / 8;
}
if (pos->bitfield_size) {
pos->byte_offset = pos->bit_offset / pos->bit_size * pos->bit_size / 8;
pos->bitfield_offset = pos->bit_offset - pos->byte_offset * 8;
if (!btfe->is_big_endian)
pos->bitfield_offset = integral_bit_size - pos->bitfield_offset - pos->bitfield_size;
pos->bitfield_offset = pos->bit_size - pos->bitfield_offset - pos->bitfield_size;
} else {
pos->byte_offset = pos->bit_offset / 8;
}
}