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Message-ID: <CAH3MdRVU5ayEm6eb9Fz53Q5gjA60vadyJ+0_meCkWBo+t+XXYA@mail.gmail.com>
Date: Thu, 14 Feb 2019 21:38:20 -0800
From: Y Song <ys114321@...il.com>
To: Joe Stringer <joe@...d.net.nz>
Cc: bpf@...r.kernel.org, netdev <netdev@...r.kernel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH bpf-next 2/4] libbpf: Support 32-bit static data loads
On Mon, Feb 11, 2019 at 4:48 PM Joe Stringer <joe@...d.net.nz> wrote:
>
> 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
A little bit clarification from compiler side.
The above compiler error is to prevent people use static variables since current
kernel/libbpf does not handle this. The compiler only warns if .bss or
.data section
has more than one definitions. The first definition always has section offset 0
and the compiler did not warn.
The restriction is a little strange. To only work with 32-bit data is
not a right
statement. The following are some examples.
The following static variable definitions will succeed:
static int a; /* one in .bss */
static long b = 2; /* one in .data */
The following definitions will fail as both in .bss.
static int a;
static int b;
The following definitions will fail as both in .data:
static char a = 2;
static int b = 3;
Using global variables can prevent compiler errors.
maps are defined as globals and the compiler does not
check whether a particular global variable is defining a map or not.
If you just use static variable like below
static int a = 2;
without potential assignment to a, the compiler will replace variable
a with 2 at compile time.
An alternative is to define like below
static volatile int a = 2;
You can get a "load" for variable "a" in the bpf load even if there is
no assignment to a.
Maybe now is the time to remove the compiler assertions as
libbpf/kernel starts to
handle static variables?
>
> Based on the proof of concept by Daniel Borkmann (presented at LPC 2018).
>
> Signed-off-by: Joe Stringer <joe@...d.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
>
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