> Am 04.09.2019 um 00:13 schrieb Daniel Borkmann <daniel@xxxxxxxxxxxxx>:
>
> On 8/16/19 12:53 PM, Ilya Leoshkevich wrote:
>> "ctx:file_pos sysctl:read write ok" fails on s390 with "Read value !=
>> nux". This is because verifier rewrites a complete 32-bit
>> bpf_sysctl.file_pos update to a partial update of the first 32 bits of
>> 64-bit *bpf_sysctl_kern.ppos, which is not correct on big-endian
>> systems.
>> Fix by using an offset on big-endian systems.
>> Ditto for bpf_sysctl.file_pos reads. Currently the test does not detect
>> a problem there, since it expects to see 0, which it gets with high
>> probability in error cases, so change it to seek to offset 3 and expect
>> 3 in bpf_sysctl.file_pos.
>> Fixes: e1550bfe0de4 ("bpf: Add file_pos field to bpf_sysctl ctx")
>> Signed-off-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
>> ---
>> v1->v2: Merge bpf_ctx_narrow_load_shift and
>> bpf_ctx_narrow_access_offset.
>> include/linux/filter.h | 8 ++++----
>> kernel/bpf/cgroup.c | 10 ++++++++--
>> kernel/bpf/verifier.c | 4 ++--
>> tools/testing/selftests/bpf/test_sysctl.c | 9 ++++++++-
>> 4 files changed, 22 insertions(+), 9 deletions(-)
>> diff --git a/include/linux/filter.h b/include/linux/filter.h
>> index 92c6e31fb008..2ce57645f3cd 100644
>> --- a/include/linux/filter.h
>> +++ b/include/linux/filter.h
>> @@ -749,14 +749,14 @@ bpf_ctx_narrow_access_ok(u32 off, u32 size, u32 size_default)
>> }
>> static inline u8
>> -bpf_ctx_narrow_load_shift(u32 off, u32 size, u32 size_default)
>> +bpf_ctx_narrow_access_offset(u32 off, u32 size, u32 size_default)
>> {
>> - u8 load_off = off & (size_default - 1);
>> + u8 access_off = off & (size_default - 1);
>> #ifdef __LITTLE_ENDIAN
>> - return load_off * 8;
>> + return access_off;
>> #else
>> - return (size_default - (load_off + size)) * 8;
>> + return size_default - (access_off + size);
>> #endif
>> }
>> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
>> index 0a00eaca6fae..00c4647ce92a 100644
>> --- a/kernel/bpf/cgroup.c
>> +++ b/kernel/bpf/cgroup.c
>> @@ -1325,6 +1325,7 @@ static u32 sysctl_convert_ctx_access(enum bpf_access_type type,
>> struct bpf_prog *prog, u32 *target_size)
>> {
>> struct bpf_insn *insn = insn_buf;
>> + u32 read_size;
>> switch (si->off) {
>> case offsetof(struct bpf_sysctl, write):
>> @@ -1356,7 +1357,9 @@ static u32 sysctl_convert_ctx_access(enum bpf_access_type type,
>> treg, si->dst_reg,
>> offsetof(struct bpf_sysctl_kern, ppos));
>> *insn++ = BPF_STX_MEM(
>> - BPF_SIZEOF(u32), treg, si->src_reg, 0);
>> + BPF_SIZEOF(u32), treg, si->src_reg,
>> + bpf_ctx_narrow_access_offset(
>> + 0, sizeof(u32), sizeof(loff_t)));
>> *insn++ = BPF_LDX_MEM(
>> BPF_DW, treg, si->dst_reg,
>> offsetof(struct bpf_sysctl_kern, tmp_reg));
>> @@ -1365,8 +1368,11 @@ static u32 sysctl_convert_ctx_access(enum bpf_access_type type,
>> BPF_FIELD_SIZEOF(struct bpf_sysctl_kern, ppos),
>> si->dst_reg, si->src_reg,
>> offsetof(struct bpf_sysctl_kern, ppos));
>> + read_size = bpf_size_to_bytes(BPF_SIZE(si->code));
>> *insn++ = BPF_LDX_MEM(
>> - BPF_SIZE(si->code), si->dst_reg, si->dst_reg, 0);
>> + BPF_SIZE(si->code), si->dst_reg, si->dst_reg,
>> + bpf_ctx_narrow_access_offset(
>> + 0, read_size, sizeof(loff_t)));
>
> I see what you're doing, but generally I'm a bit puzzled on why we need these
> partial store/loads and cannot access the full loff_t value internally with the
> rewrite. Why was BPF_SIZEOF(u32) chosen in the first place? Looks like git history
> doesn't have any useful insight here ... Andrey mind to put some clarifications
> on this? Thx
Do you mean rewriting e.g. the following:
BPF_LDX_MEM(access_size, si->dst_reg, si->src_reg,
offsetof(struct bpf_sysctl, file_pos)));
as the following?
BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct bpf_sysctl_kern, ppos),
si->dst_reg, si->src_reg,
offsetof(struct bpf_sysctl_kern, ppos));
BPF_LDX_MEM(BPF_DW, si->dst_reg, si->dst_reg, 0); # full loff_t
while also doing
*target_size = sizeof(loff_t);
in order to let is_narrower_load logic in convert_ctx_accesses () do the
masking? That would work, but wouldn't the generated code be less
efficient due to explicit masking?
