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Yonghong Song 581738a681 bpf: Provide better register bounds after jmp32 instructions 
With latest llvm (trunk https://github.com/llvm/llvm-project),
test_progs, which has +alu32 enabled, failed for strobemeta.o.
The verifier output looks like below with edit to replace large
decimal numbers with hex ones.
 193: (85) call bpf_probe_read_user_str#114
   R0=inv(id=0)
 194: (26) if w0 > 0x1 goto pc+4
   R0_w=inv(id=0,umax_value=0xffffffff00000001)
 195: (6b) *(u16 *)(r7 +80) = r0
 196: (bc) w6 = w0
   R6_w=inv(id=0,umax_value=0xffffffff,var_off=(0x0; 0xffffffff))
 197: (67) r6 <<= 32
   R6_w=inv(id=0,smax_value=0x7fffffff00000000,umax_value=0xffffffff00000000,
            var_off=(0x0; 0xffffffff00000000))
 198: (77) r6 >>= 32
   R6=inv(id=0,umax_value=0xffffffff,var_off=(0x0; 0xffffffff))
 ...
 201: (79) r8 = *(u64 *)(r10 -416)
   R8_w=map_value(id=0,off=40,ks=4,vs=13872,imm=0)
 202: (0f) r8 += r6
   R8_w=map_value(id=0,off=40,ks=4,vs=13872,umax_value=0xffffffff,var_off=(0x0; 0xffffffff))
 203: (07) r8 += 9696
   R8_w=map_value(id=0,off=9736,ks=4,vs=13872,umax_value=0xffffffff,var_off=(0x0; 0xffffffff))
 ...
 255: (bf) r1 = r8
   R1_w=map_value(id=0,off=9736,ks=4,vs=13872,umax_value=0xffffffff,var_off=(0x0; 0xffffffff))
 ...
 257: (85) call bpf_probe_read_user_str#114
 R1 unbounded memory access, make sure to bounds check any array access into a map

The value range for register r6 at insn 198 should be really just 0/1.
The umax_value=0xffffffff caused later verification failure.

After jmp instructions, the current verifier already tried to use just
obtained information to get better register range. The current mechanism is
for 64bit register only. This patch implemented to tighten the range
for 32bit sub-registers after jmp32 instructions.
With the patch, we have the below range ranges for the
above code sequence:
 193: (85) call bpf_probe_read_user_str#114
   R0=inv(id=0)
 194: (26) if w0 > 0x1 goto pc+4
   R0_w=inv(id=0,smax_value=0x7fffffff00000001,umax_value=0xffffffff00000001,
            var_off=(0x0; 0xffffffff00000001))
 195: (6b) *(u16 *)(r7 +80) = r0
 196: (bc) w6 = w0
   R6_w=inv(id=0,umax_value=0xffffffff,var_off=(0x0; 0x1))
 197: (67) r6 <<= 32
   R6_w=inv(id=0,umax_value=0x100000000,var_off=(0x0; 0x100000000))
 198: (77) r6 >>= 32
   R6=inv(id=0,umax_value=1,var_off=(0x0; 0x1))
 ...
 201: (79) r8 = *(u64 *)(r10 -416)
   R8_w=map_value(id=0,off=40,ks=4,vs=13872,imm=0)
 202: (0f) r8 += r6
   R8_w=map_value(id=0,off=40,ks=4,vs=13872,umax_value=1,var_off=(0x0; 0x1))
 203: (07) r8 += 9696
   R8_w=map_value(id=0,off=9736,ks=4,vs=13872,umax_value=1,var_off=(0x0; 0x1))
 ...
 255: (bf) r1 = r8
   R1_w=map_value(id=0,off=9736,ks=4,vs=13872,umax_value=1,var_off=(0x0; 0x1))
 ...
 257: (85) call bpf_probe_read_user_str#114
 ...

At insn 194, the register R0 has better var_off.mask and smax_value.
Especially, the var_off.mask ensures later lshift and rshift
maintains proper value range.

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191121170650.449030-1-yhs@fb.com
2019-11-24 16:58:46 -08:00
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README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.