tools/lib/bpf/bpf_core_read.h

   1 /* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
   2 #ifndef __BPF_CORE_READ_H__
   3 #define __BPF_CORE_READ_H__
   4
   5 /*
   6  * enum bpf_field_info_kind is passed as a second argument into
   7  * __builtin_preserve_field_info() built-in to get a specific aspect of
   8  * a field, captured as a first argument. __builtin_preserve_field_info(field,
   9  * info_kind) returns __u32 integer and produces BTF field relocation, which
  10  * is understood and processed by libbpf during BPF object loading. See
  11  * selftests/bpf for examples.
  12  */
  13 enum bpf_field_info_kind {
  14         BPF_FIELD_BYTE_OFFSET = 0,      /* field byte offset */
  15         BPF_FIELD_BYTE_SIZE = 1,
  16         BPF_FIELD_EXISTS = 2,           /* field existence in target kernel */
  17         BPF_FIELD_SIGNED = 3,
  18         BPF_FIELD_LSHIFT_U64 = 4,
  19         BPF_FIELD_RSHIFT_U64 = 5,
  20 };
  21
  22 #define __CORE_RELO(src, field, info)                                         \
  23         __builtin_preserve_field_info((src)->field, BPF_FIELD_##info)
  24
  25 #if __BYTE_ORDER == __LITTLE_ENDIAN
  26 #define __CORE_BITFIELD_PROBE_READ(dst, src, fld)                             \
  27         bpf_probe_read((void *)dst,                                           \
  28                        __CORE_RELO(src, fld, BYTE_SIZE),                      \
  29                        (const void *)src + __CORE_RELO(src, fld, BYTE_OFFSET))
  30 #else
  31 /* semantics of LSHIFT_64 assumes loading values into low-ordered bytes, so
  32  * for big-endian we need to adjust destination pointer accordingly, based on
  33  * field byte size
  34  */
  35 #define __CORE_BITFIELD_PROBE_READ(dst, src, fld)                             \
  36         bpf_probe_read((void *)dst + (8 - __CORE_RELO(src, fld, BYTE_SIZE)),  \
  37                        __CORE_RELO(src, fld, BYTE_SIZE),                      \
  38                        (const void *)src + __CORE_RELO(src, fld, BYTE_OFFSET))
  39 #endif
  40
  41 /*
  42  * Extract bitfield, identified by s->field, and return its value as u64.
  43  * All this is done in relocatable manner, so bitfield changes such as
  44  * signedness, bit size, offset changes, this will be handled automatically.
  45  * This version of macro is using bpf_probe_read() to read underlying integer
  46  * storage. Macro functions as an expression and its return type is
  47  * bpf_probe_read()'s return value: 0, on success, <0 on error.
  48  */
  49 #define BPF_CORE_READ_BITFIELD_PROBED(s, field) ({                            \
  50         unsigned long long val = 0;                                           \
  51                                                                               \
  52         __CORE_BITFIELD_PROBE_READ(&val, s, field);                           \
  53         val <<= __CORE_RELO(s, field, LSHIFT_U64);                            \
  54         if (__CORE_RELO(s, field, SIGNED))                                    \
  55                 val = ((long long)val) >> __CORE_RELO(s, field, RSHIFT_U64);  \
  56         else                                                                  \
  57                 val = val >> __CORE_RELO(s, field, RSHIFT_U64);               \
  58         val;                                                                  \
  59 })
  60
  61 /*
  62  * Extract bitfield, identified by s->field, and return its value as u64.
  63  * This version of macro is using direct memory reads and should be used from
  64  * BPF program types that support such functionality (e.g., typed raw
  65  * tracepoints).
  66  */
  67 #define BPF_CORE_READ_BITFIELD(s, field) ({                                   \
  68         const void *p = (const void *)s + __CORE_RELO(s, field, BYTE_OFFSET); \
  69         unsigned long long val;                                               \
  70                                                                               \
  71         switch (__CORE_RELO(s, field, BYTE_SIZE)) {                           \
  72         case 1: val = *(const unsigned char *)p;                              \
  73         case 2: val = *(const unsigned short *)p;                             \
  74         case 4: val = *(const unsigned int *)p;                               \
  75         case 8: val = *(const unsigned long long *)p;                         \
  76         }                                                                     \
  77         val <<= __CORE_RELO(s, field, LSHIFT_U64);                            \
  78         if (__CORE_RELO(s, field, SIGNED))                                    \
  79                 val = ((long long)val) >> __CORE_RELO(s, field, RSHIFT_U64);  \
  80         else                                                                  \
  81                 val = val >> __CORE_RELO(s, field, RSHIFT_U64);               \
  82         val;                                                                  \
  83 })
  84
  85 /*
  86  * Convenience macro to check that field actually exists in target kernel's.
  87  * Returns:
  88  *    1, if matching field is present in target kernel;
  89  *    0, if no matching field found.
  90  */
  91 #define bpf_core_field_exists(field)                                        \
  92         __builtin_preserve_field_info(field, BPF_FIELD_EXISTS)
  93
  94 /*
  95  * Convenience macro to get byte size of a field. Works for integers,
  96  * struct/unions, pointers, arrays, and enums.
  97  */
  98 #define bpf_core_field_size(field)                                          \
  99         __builtin_preserve_field_info(field, BPF_FIELD_BYTE_SIZE)
 100
 101 /*
 102  * bpf_core_read() abstracts away bpf_probe_read() call and captures offset
 103  * relocation for source address using __builtin_preserve_access_index()
 104  * built-in, provided by Clang.
 105  *
 106  * __builtin_preserve_access_index() takes as an argument an expression of
 107  * taking an address of a field within struct/union. It makes compiler emit
 108  * a relocation, which records BTF type ID describing root struct/union and an
 109  * accessor string which describes exact embedded field that was used to take
 110  * an address. See detailed description of this relocation format and
 111  * semantics in comments to struct bpf_field_reloc in libbpf_internal.h.
 112  *
 113  * This relocation allows libbpf to adjust BPF instruction to use correct
 114  * actual field offset, based on target kernel BTF type that matches original
 115  * (local) BTF, used to record relocation.
 116  */
 117 #define bpf_core_read(dst, sz, src)                                         \
 118         bpf_probe_read(dst, sz,                                             \
 119                        (const void *)__builtin_preserve_access_index(src))
 120
 121 /*
 122  * bpf_core_read_str() is a thin wrapper around bpf_probe_read_str()
 123  * additionally emitting BPF CO-RE field relocation for specified source
 124  * argument.
 125  */
 126 #define bpf_core_read_str(dst, sz, src)                                     \
 127         bpf_probe_read_str(dst, sz,                                         \
 128                            (const void *)__builtin_preserve_access_index(src))
 129
 130 #define ___concat(a, b) a ## b
 131 #define ___apply(fn, n) ___concat(fn, n)
 132 #define ___nth(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, __11, N, ...) N
 133
 134 /*
 135  * return number of provided arguments; used for switch-based variadic macro
 136  * definitions (see ___last, ___arrow, etc below)
 137  */
 138 #define ___narg(...) ___nth(_, ##__VA_ARGS__, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
 139 /*
 140  * return 0 if no arguments are passed, N - otherwise; used for
 141  * recursively-defined macros to specify termination (0) case, and generic
 142  * (N) case (e.g., ___read_ptrs, ___core_read)
 143  */
 144 #define ___empty(...) ___nth(_, ##__VA_ARGS__, N, N, N, N, N, N, N, N, N, N, 0)
 145
 146 #define ___last1(x) x
 147 #define ___last2(a, x) x
 148 #define ___last3(a, b, x) x
 149 #define ___last4(a, b, c, x) x
 150 #define ___last5(a, b, c, d, x) x
 151 #define ___last6(a, b, c, d, e, x) x
 152 #define ___last7(a, b, c, d, e, f, x) x
 153 #define ___last8(a, b, c, d, e, f, g, x) x
 154 #define ___last9(a, b, c, d, e, f, g, h, x) x
 155 #define ___last10(a, b, c, d, e, f, g, h, i, x) x
 156 #define ___last(...) ___apply(___last, ___narg(__VA_ARGS__))(__VA_ARGS__)
 157
 158 #define ___nolast2(a, _) a
 159 #define ___nolast3(a, b, _) a, b
 160 #define ___nolast4(a, b, c, _) a, b, c
 161 #define ___nolast5(a, b, c, d, _) a, b, c, d
 162 #define ___nolast6(a, b, c, d, e, _) a, b, c, d, e
 163 #define ___nolast7(a, b, c, d, e, f, _) a, b, c, d, e, f
 164 #define ___nolast8(a, b, c, d, e, f, g, _) a, b, c, d, e, f, g
 165 #define ___nolast9(a, b, c, d, e, f, g, h, _) a, b, c, d, e, f, g, h
 166 #define ___nolast10(a, b, c, d, e, f, g, h, i, _) a, b, c, d, e, f, g, h, i
 167 #define ___nolast(...) ___apply(___nolast, ___narg(__VA_ARGS__))(__VA_ARGS__)
 168
 169 #define ___arrow1(a) a
 170 #define ___arrow2(a, b) a->b
 171 #define ___arrow3(a, b, c) a->b->c
 172 #define ___arrow4(a, b, c, d) a->b->c->d
 173 #define ___arrow5(a, b, c, d, e) a->b->c->d->e
 174 #define ___arrow6(a, b, c, d, e, f) a->b->c->d->e->f
 175 #define ___arrow7(a, b, c, d, e, f, g) a->b->c->d->e->f->g
 176 #define ___arrow8(a, b, c, d, e, f, g, h) a->b->c->d->e->f->g->h
 177 #define ___arrow9(a, b, c, d, e, f, g, h, i) a->b->c->d->e->f->g->h->i
 178 #define ___arrow10(a, b, c, d, e, f, g, h, i, j) a->b->c->d->e->f->g->h->i->j
 179 #define ___arrow(...) ___apply(___arrow, ___narg(__VA_ARGS__))(__VA_ARGS__)
 180
 181 #define ___type(...) typeof(___arrow(__VA_ARGS__))
 182
 183 #define ___read(read_fn, dst, src_type, src, accessor)                      \
 184         read_fn((void *)(dst), sizeof(*(dst)), &((src_type)(src))->accessor)
 185
 186 /* "recursively" read a sequence of inner pointers using local __t var */
 187 #define ___rd_first(src, a) ___read(bpf_core_read, &__t, ___type(src), src, a);
 188 #define ___rd_last(...)                                                     \
 189         ___read(bpf_core_read, &__t,                                        \
 190                 ___type(___nolast(__VA_ARGS__)), __t, ___last(__VA_ARGS__));
 191 #define ___rd_p1(...) const void *__t; ___rd_first(__VA_ARGS__)
 192 #define ___rd_p2(...) ___rd_p1(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
 193 #define ___rd_p3(...) ___rd_p2(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
 194 #define ___rd_p4(...) ___rd_p3(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
 195 #define ___rd_p5(...) ___rd_p4(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
 196 #define ___rd_p6(...) ___rd_p5(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
 197 #define ___rd_p7(...) ___rd_p6(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
 198 #define ___rd_p8(...) ___rd_p7(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
 199 #define ___rd_p9(...) ___rd_p8(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
 200 #define ___read_ptrs(src, ...)                                              \
 201         ___apply(___rd_p, ___narg(__VA_ARGS__))(src, __VA_ARGS__)
 202
 203 #define ___core_read0(fn, dst, src, a)                                      \
 204         ___read(fn, dst, ___type(src), src, a);
 205 #define ___core_readN(fn, dst, src, ...)                                    \
 206         ___read_ptrs(src, ___nolast(__VA_ARGS__))                           \
 207         ___read(fn, dst, ___type(src, ___nolast(__VA_ARGS__)), __t,         \
 208                 ___last(__VA_ARGS__));
 209 #define ___core_read(fn, dst, src, a, ...)                                  \
 210         ___apply(___core_read, ___empty(__VA_ARGS__))(fn, dst,              \
 211                                                       src, a, ##__VA_ARGS__)
 212
 213 /*
 214  * BPF_CORE_READ_INTO() is a more performance-conscious variant of
 215  * BPF_CORE_READ(), in which final field is read into user-provided storage.
 216  * See BPF_CORE_READ() below for more details on general usage.
 217  */
 218 #define BPF_CORE_READ_INTO(dst, src, a, ...)                                \
 219         ({                                                                  \
 220                 ___core_read(bpf_core_read, dst, src, a, ##__VA_ARGS__)     \
 221         })
 222
 223 /*
 224  * BPF_CORE_READ_STR_INTO() does same "pointer chasing" as
 225  * BPF_CORE_READ() for intermediate pointers, but then executes (and returns
 226  * corresponding error code) bpf_core_read_str() for final string read.
 227  */
 228 #define BPF_CORE_READ_STR_INTO(dst, src, a, ...)                            \
 229         ({                                                                  \
 230                 ___core_read(bpf_core_read_str, dst, src, a, ##__VA_ARGS__) \
 231         })
 232
 233 /*
 234  * BPF_CORE_READ() is used to simplify BPF CO-RE relocatable read, especially
 235  * when there are few pointer chasing steps.
 236  * E.g., what in non-BPF world (or in BPF w/ BCC) would be something like:
 237  *      int x = s->a.b.c->d.e->f->g;
 238  * can be succinctly achieved using BPF_CORE_READ as:
 239  *      int x = BPF_CORE_READ(s, a.b.c, d.e, f, g);
 240  *
 241  * BPF_CORE_READ will decompose above statement into 4 bpf_core_read (BPF
 242  * CO-RE relocatable bpf_probe_read() wrapper) calls, logically equivalent to:
 243  * 1. const void *__t = s->a.b.c;
 244  * 2. __t = __t->d.e;
 245  * 3. __t = __t->f;
 246  * 4. return __t->g;
 247  *
 248  * Equivalence is logical, because there is a heavy type casting/preservation
 249  * involved, as well as all the reads are happening through bpf_probe_read()
 250  * calls using __builtin_preserve_access_index() to emit CO-RE relocations.
 251  *
 252  * N.B. Only up to 9 "field accessors" are supported, which should be more
 253  * than enough for any practical purpose.
 254  */
 255 #define BPF_CORE_READ(src, a, ...)                                          \
 256         ({                                                                  \
 257                 ___type(src, a, ##__VA_ARGS__) __r;                         \
 258                 BPF_CORE_READ_INTO(&__r, src, a, ##__VA_ARGS__);            \
 259                 __r;                                                        \
 260         })
 261
 262 #endif
 263