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bpf: permit map_ptr arithmetic with opcode add and offset 0 #9

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bpf: permit map_ptr arithmetic with opcode add and offset 0 #9

wants to merge 3 commits into from

Commits on Sep 6, 2020

  1. adding ci files

    @tsipa
    tsipa committed Sep 6, 2020
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  2. Commit 41c48f3 ("bpf: Support access 

    to bpf map fields") added support to access map fields
with CORE support. For example,

            struct bpf_map {
                    __u32 max_entries;
            } __attribute__((preserve_access_index));

            struct bpf_array {
                    struct bpf_map map;
                    __u32 elem_size;
            } __attribute__((preserve_access_index));

            struct {
                    __uint(type, BPF_MAP_TYPE_ARRAY);
                    __uint(max_entries, 4);
                    __type(key, __u32);
                    __type(value, __u32);
            } m_array SEC(".maps");

            SEC("cgroup_skb/egress")
            int cg_skb(void *ctx)
            {
                    struct bpf_array *array = (struct bpf_array *)&m_array;

                    /* .. array->map.max_entries .. */
            }

In kernel, bpf_htab has similar structure,

	    struct bpf_htab {
		    struct bpf_map map;
                    ...
            }

In the above cg_skb(), to access array->map.max_entries, with CORE, the clang will
generate two builtin's.
            base = &m_array;
            /* access array.map */
            map_addr = __builtin_preserve_struct_access_info(base, 0, 0);
            /* access array.map.max_entries */
            max_entries_addr = __builtin_preserve_struct_access_info(map_addr, 0, 0);
	    max_entries = *max_entries_addr;

In the current llvm, if two builtin's are in the same function or
in the same function after inlining, the compiler is smart enough to chain
them together and generates like below:
            base = &m_array;
            max_entries = *(base + reloc_offset); /* reloc_offset = 0 in this case */
and we are fine.

But if we force no inlining for one of functions in test_map_ptr() selftest, e.g.,
check_default(), the above two __builtin_preserve_* will be in two different
functions. In this case, we will have code like:
   func check_hash():
            reloc_offset_map = 0;
            base = &m_array;
            map_base = base + reloc_offset_map;
            check_default(map_base, ...)
   func check_default(map_base, ...):
            max_entries = *(map_base + reloc_offset_max_entries);

In kernel, map_ptr (CONST_PTR_TO_MAP) does not allow any arithmetic.
The above "map_base = base + reloc_offset_map" will trigger a verifier failure.
  ; VERIFY(check_default(&hash->map, map));
  0: (18) r7 = 0xffffb4fe8018a004
  2: (b4) w1 = 110
  3: (63) *(u32 *)(r7 +0) = r1
   R1_w=invP110 R7_w=map_value(id=0,off=4,ks=4,vs=8,imm=0) R10=fp0
  ; VERIFY_TYPE(BPF_MAP_TYPE_HASH, check_hash);
  4: (18) r1 = 0xffffb4fe8018a000
  6: (b4) w2 = 1
  7: (63) *(u32 *)(r1 +0) = r2
   R1_w=map_value(id=0,off=0,ks=4,vs=8,imm=0) R2_w=invP1 R7_w=map_value(id=0,off=4,ks=4,vs=8,imm=0) R10=fp0
  8: (b7) r2 = 0
  9: (18) r8 = 0xffff90bcb500c000
  11: (18) r1 = 0xffff90bcb500c000
  13: (0f) r1 += r2
  R1 pointer arithmetic on map_ptr prohibited

To fix the issue, let us permit map_ptr + 0 arithmetic which will
result in exactly the same map_ptr.

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 kernel/bpf/verifier.c | 3 +++
 1 file changed, 3 insertions(+)
    @yonghong-song @tsipa
    yonghong-song authored and tsipa committed Sep 6, 2020
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  3. change selftest map_ptr_kern.c which will fail without previous 

    verifier change. Also added to verifier test for both
"map_ptr += scalar" and "scalar += map_ptr" arithmetic.

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 .../selftests/bpf/progs/map_ptr_kern.c        |  4 +--
 .../testing/selftests/bpf/verifier/map_ptr.c  | 32 +++++++++++++++++++
 2 files changed, 34 insertions(+), 2 deletions(-)
    @yonghong-song @tsipa
    yonghong-song authored and tsipa committed Sep 6, 2020
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