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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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