mirros/rockchip-kernel
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

bpf/verifier: improve register value range tracking with ARSH

Browse Source 
When helpers like bpf_get_stack returns an int value
and later on used for arithmetic computation, the LSH and ARSH
operations are often required to get proper sign extension into
64-bit. For example, without this patch:
    54: R0=inv(id=0,umax_value=800)
    54: (bf) r8 = r0
    55: R0=inv(id=0,umax_value=800) R8_w=inv(id=0,umax_value=800)
    55: (67) r8 <<= 32
    56: R8_w=inv(id=0,umax_value=3435973836800,var_off=(0x0; 0x3ff00000000))
    56: (c7) r8 s>>= 32
    57: R8=inv(id=0)
With this patch:
    54: R0=inv(id=0,umax_value=800)
    54: (bf) r8 = r0
    55: R0=inv(id=0,umax_value=800) R8_w=inv(id=0,umax_value=800)
    55: (67) r8 <<= 32
    56: R8_w=inv(id=0,umax_value=3435973836800,var_off=(0x0; 0x3ff00000000))
    56: (c7) r8 s>>= 32
    57: R8=inv(id=0, umax_value=800,var_off=(0x0; 0x3ff))
With better range of "R8", later on when "R8" is added to other register,
e.g., a map pointer or scalar-value register, the better register
range can be derived and verifier failure may be avoided.

In our later example,
    ......
    usize = bpf_get_stack(ctx, raw_data, max_len, BPF_F_USER_STACK);
    if (usize < 0)
        return 0;
    ksize = bpf_get_stack(ctx, raw_data + usize, max_len - usize, 0);
    ......
Without improving ARSH value range tracking, the register representing
"max_len - usize" will have smin_value equal to S64_MIN and will be
rejected by verifier.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Yonghong Song
2018-04-28 22:28:11 -07:00
committed by Alexei Starovoitov
parent afbe1a5b79
commit 9cbe1f5a32
3 changed files with 36 additions and 1 deletions
Download Patch File Download Diff File Expand all files Collapse all files

4
include/linux/tnum.h
View File

@@ -23,8 +23,10 @@ struct tnum tnum_range(u64 min, u64 max);
/* Arithmetic and logical ops */
/* Shift a tnum left (by a fixed shift) */
struct tnum tnum_lshift(struct tnum a, u8 shift);
/* Shift a tnum right (by a fixed shift) */
/* Shift (rsh) a tnum right (by a fixed shift) */
struct tnum tnum_rshift(struct tnum a, u8 shift);
/* Shift (arsh) a tnum right (by a fixed min_shift) */
struct tnum tnum_arshift(struct tnum a, u8 min_shift);
/* Add two tnums, return @a + @b */
struct tnum tnum_add(struct tnum a, struct tnum b);
/* Subtract two tnums, return @a - @b */