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bpf: change bpf_probe_read_str arg2 type to ARG_CONST_SIZE_OR_ZERO

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Commit 9fd29c08e520 ("bpf: improve verifier ARG_CONST_SIZE_OR_ZERO
semantics") relaxed the treatment of ARG_CONST_SIZE_OR_ZERO due to the way
the compiler generates optimized BPF code when checking boundaries of an
argument from C code. A typical example of this optimized code can be
generated using the bpf_probe_read_str helper when operating on variable
memory:

/* len is a generic scalar */
if (len > 0 && len <= 0x7fff)
        bpf_probe_read_str(p, len, s);

251: (79) r1 = *(u64 *)(r10 -88)
252: (07) r1 += -1
253: (25) if r1 > 0x7ffe goto pc-42
254: (bf) r1 = r7
255: (79) r2 = *(u64 *)(r10 -88)
256: (bf) r8 = r4
257: (85) call bpf_probe_read_str#45
R2 min value is negative, either use unsigned or 'var &= const'

With this code, the verifier loses track of the variable.

Replacing arg2 with ARG_CONST_SIZE_OR_ZERO is thus desirable since it
avoids this quite common case which leads to usability issues, and the
compiler generates code that the verifier can more easily test:

if (len <= 0x7fff)
        bpf_probe_read_str(p, len, s);

or

bpf_probe_read_str(p, len & 0x7fff, s);

No changes to the bpf_probe_read_str helper are necessary since
strncpy_from_unsafe itself immediately returns if the size passed is 0.

Signed-off-by: Gianluca Borello <g.borello@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
This commit is contained in:
Gianluca Borello 2017-11-22 18:32:55 +00:00 committed by Daniel Borkmann
parent eb33f2cca4
commit 5c4e120174
1 changed files with 1 additions and 1 deletions
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2
kernel/trace/bpf_trace.c
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@ -494,7 +494,7 @@ static const struct bpf_func_proto bpf_probe_read_str_proto = {
.gpl_only = true, .gpl_only = true,
.ret_type = RET_INTEGER, .ret_type = RET_INTEGER,
.arg1_type = ARG_PTR_TO_UNINIT_MEM, .arg1_type = ARG_PTR_TO_UNINIT_MEM,
.arg2_type = ARG_CONST_SIZE, .arg2_type = ARG_CONST_SIZE_OR_ZERO,
.arg3_type = ARG_ANYTHING, .arg3_type = ARG_ANYTHING,
}; };