Non const src shift#12382
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Currently, the BPF verifier only allows shift operations when the shift
amount is a known constant. This is overly restrictive for cases where
the shift amount is bounded but not fully determined at verification time.
For example, the following code is rejected by the verifier even though
the shift amount is bounded to [1, 4]:
u32 shift = bpf_get_prandom_u32();
shift &= 3; // shift is in range [0, 3]
shift += 1; // shift is in range [1, 4]
r1 <<= shift; // non-const but bounded shift amount
Modify the shift helper functions (scalar_min_max_lsh, scalar32_min_max_lsh,
scalar_min_max_rsh, scalar32_min_max_rsh, scalar_min_max_arsh,
scalar32_min_max_arsh) to handle non-const but bounded shift amounts.
Updated is_safe_to_compute_dst_reg_range() to allow shift operations
when the shift amount is within the valid range (< 32 for 32-bit shifts,
< 64 for 64-bit shifts), rather than requiring it to be a constant.
This approach ensures the verifier remains sound while allowing more
programs to pass verification.
Also modify the comment on is_safe_to_compute_dst_reg_range.
Shifts by more than insn bitness are legal in the BPF ISA; they are
implementation-defined behaviour [of the underlying architecture],
rather than UB, and have been made legal for performance reasons.
Add test cases for shift operations with non-const but bounded source operand: - shift_with_non_const_src_lsh: Tests left shift (BPF_LSH) where the shift amount is in range [1, 4] and the destination is a known constant (1). The verifier should compute correct bounds [2, 16] for the result. - shift_with_non_const_src_rsh: Tests logical right shift (BPF_RSH) where the shift amount is in range [1, 4] and the destination is 0xff. The verifier should compute correct bounds [15, 127] for the result. - shift_with_non_const_src_arsh: Tests arithmetic right shift (BPF_ARSH) where the shift amount is in range [1, 4] and the destination is a negative constant (-8). The verifier applies the three-branch signed bound logic to derive result bounds [-4, -1]. When the shift amount is non-constant, the var_off is conservatively set to tnum_unknown.
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