Support loads of static 32-bit data when BPF writers make use of
convenience macros for accessing static global data variables. A later
patch in this series will demonstrate its usage in a selftest.
As of LLVM-7, this technique only works with 32-bit data, as LLVM will
complain if this technique is attempted with data of other sizes:
LLVM ERROR: Unsupported relocation: try to compile with -O2 or above,
or check your static variable usage
Based on the proof of concept by Daniel Borkmann (presented at LPC 2018).
Signed-off-by: Joe Stringer <j...@wand.net.nz>
---
tools/lib/bpf/libbpf.c | 34 ++++++++++++++++++++++++++++++++--
1 file changed, 32 insertions(+), 2 deletions(-)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 1ec28d5154dc..da35d5559b22 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -140,11 +140,13 @@ struct bpf_program {
enum {
RELO_LD64,
RELO_CALL,
+ RELO_DATA,
} type;
int insn_idx;
union {
int map_idx;
int text_off;
+ uint32_t data;
};
} *reloc_desc;
int nr_reloc;
@@ -210,6 +212,7 @@ struct bpf_object {
Elf *elf;
GElf_Ehdr ehdr;
Elf_Data *symbols;
+ Elf_Data *global_data;
size_t strtabidx;
struct {
GElf_Shdr shdr;
@@ -218,6 +221,7 @@ struct bpf_object {
int nr_reloc;
int maps_shndx;
int text_shndx;
+ int data_shndx;
} efile;
/*
* All loaded bpf_object is linked in a list, which is
@@ -476,6 +480,7 @@ static void bpf_object__elf_finish(struct bpf_object *obj)
obj->efile.elf = NULL;
}
obj->efile.symbols = NULL;
+ obj->efile.global_data = NULL;
zfree(&obj->efile.reloc);
obj->efile.nr_reloc = 0;
@@ -866,6 +871,9 @@ static int bpf_object__elf_collect(struct bpf_object *obj,
int flags)
pr_warning("failed to alloc program %s
(%s): %s",
name, obj->path, cp);
}
+ } else if (strcmp(name, ".data") == 0) {
+ obj->efile.global_data = data;
+ obj->efile.data_shndx = idx;
}
} else if (sh.sh_type == SHT_REL) {
void *reloc = obj->efile.reloc;
@@ -962,6 +970,7 @@ bpf_program__collect_reloc(struct bpf_program *prog,
GElf_Shdr *shdr,
Elf_Data *symbols = obj->efile.symbols;
int text_shndx = obj->efile.text_shndx;
int maps_shndx = obj->efile.maps_shndx;
+ int data_shndx = obj->efile.data_shndx;
struct bpf_map *maps = obj->maps;
size_t nr_maps = obj->nr_maps;
int i, nrels;
@@ -1000,8 +1009,9 @@ bpf_program__collect_reloc(struct bpf_program *prog,
GElf_Shdr *shdr,
(long long) (rel.r_info >> 32),
(long long) sym.st_value, sym.st_name);
- if (sym.st_shndx != maps_shndx && sym.st_shndx != text_shndx) {
- pr_warning("Program '%s' contains non-map related relo
data pointing to section %u\n",
+ if (sym.st_shndx != maps_shndx && sym.st_shndx != text_shndx &&
+ sym.st_shndx != data_shndx) {
+ pr_warning("Program '%s' contains unrecognized relo
data pointing to section %u\n",
prog->section_name, sym.st_shndx);
return -LIBBPF_ERRNO__RELOC;
}
@@ -1046,6 +1056,20 @@ bpf_program__collect_reloc(struct bpf_program *prog,
GElf_Shdr *shdr,
prog->reloc_desc[i].type = RELO_LD64;
prog->reloc_desc[i].insn_idx = insn_idx;
prog->reloc_desc[i].map_idx = map_idx;
+ } else if (sym.st_shndx == data_shndx) {
+ Elf_Data *global_data = obj->efile.global_data;
+ uint32_t *static_data;
+
+ if (sym.st_value + sizeof(uint32_t) >
(int)global_data->d_size) {
+ pr_warning("bpf relocation: static data load
beyond data size %lu\n",
+ global_data->d_size);
+ return -LIBBPF_ERRNO__RELOC;
+ }
+
+ static_data = global_data->d_buf + sym.st_value;
+ prog->reloc_desc[i].type = RELO_DATA;
+ prog->reloc_desc[i].insn_idx = insn_idx;
+ prog->reloc_desc[i].data = *static_data;
}
}
return 0;
@@ -1399,6 +1423,12 @@ bpf_program__relocate(struct bpf_program *prog, struct
bpf_object *obj)
&prog->reloc_desc[i]);
if (err)
return err;
+ } else if (prog->reloc_desc[i].type == RELO_DATA) {
+ struct bpf_insn *insns = prog->insns;
+ int insn_idx;
+
+ insn_idx = prog->reloc_desc[i].insn_idx;
+ insns[insn_idx].imm = prog->reloc_desc[i].data;
}
}
--
2.19.1
