Commit 4621202a authored by Andrii Nakryiko's avatar Andrii Nakryiko Committed by Alexei Starovoitov
Browse files

bpf: generalize reg_set_min_max() to handle two sets of two registers 



Change reg_set_min_max() to take FALSE/TRUE sets of two registers each,
instead of assuming that we are always comparing to a constant. For now
we still assume that right-hand side registers are constants (and make
sure that's the case by swapping src/dst regs, if necessary), but
subsequent patches will remove this limitation.

reg_set_min_max() is now called unconditionally for any register
comparison, so that might include pointer vs pointer. This makes it
consistent with is_branch_taken() generality. But we currently only
support adjustments based on SCALAR vs SCALAR comparisons, so
reg_set_min_max() has to guard itself againts pointers.

Taking two by two registers allows to further unify and simplify
check_cond_jmp_op() logic. We utilize fake register for BPF_K
conditional jump case, just like with is_branch_taken() part.

Acked-by: default avatarEduard Zingerman <eddyz87@gmail.com>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231102033759.2541186-18-andrii@kernel.org


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent 811476e9

kernel/bpf/verifier.c

+56 −75
Original line number Diff line number Diff line
@@ -14400,32 +14400,50 @@ static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state *reg 
	return is_scalar_branch_taken(reg1, reg2, opcode, is_jmp32);

}









/* Adjusts the register min/max values in the case that the dst_reg is the









 * variable register that we are working on, and src_reg is a constant or we're









 * simply doing a BPF_K check.









 * In JEQ/JNE cases we also adjust the var_off values.









/* Adjusts the register min/max values in the case that the dst_reg and









 * src_reg are both SCALAR_VALUE registers (or we are simply doing a BPF_K









 * check, in which case we havea fake SCALAR_VALUE representing insn->imm).









 * Technically we can do similar adjustments for pointers to the same object,









 * but we don't support that right now.









 */









static void reg_set_min_max(struct bpf_reg_state *true_reg1,









			    struct bpf_reg_state *true_reg2,









			    struct bpf_reg_state *false_reg1,









			    u64 uval, u32 uval32,









			    struct bpf_reg_state *false_reg2,









			    u8 opcode, bool is_jmp32)









{









	struct tnum false_32off = tnum_subreg(false_reg1->var_off);









	struct tnum false_64off = false_reg1->var_off;









	struct tnum true_32off = tnum_subreg(true_reg1->var_off);









	struct tnum true_64off = true_reg1->var_off;









	s64 sval = (s64)uval;









	s32 sval32 = (s32)uval32;









	struct tnum false_32off, false_64off;









	struct tnum true_32off, true_64off;









	u64 uval;









	u32 uval32;









	s64 sval;









	s32 sval32;

















	/* If the dst_reg is a pointer, we can't learn anything about its









	 * variable offset from the compare (unless src_reg were a pointer into









	 * the same object, but we don't bother with that.









	 * Since false_reg1 and true_reg1 have the same type by construction, we









	 * only need to check one of them for pointerness.









	/* If either register is a pointer, we can't learn anything about its









	 * variable offset from the compare (unless they were a pointer into









	 * the same object, but we don't bother with that).









	 */









	if (__is_pointer_value(false, false_reg1))









	if (false_reg1->type != SCALAR_VALUE || false_reg2->type != SCALAR_VALUE)









		return;

















	/* we expect right-hand registers (src ones) to be constants, for now */









	if (!is_reg_const(false_reg2, is_jmp32)) {









		opcode = flip_opcode(opcode);









		swap(true_reg1, true_reg2);









		swap(false_reg1, false_reg2);









	}









	if (!is_reg_const(false_reg2, is_jmp32))









		return;

















	false_32off = tnum_subreg(false_reg1->var_off);









	false_64off = false_reg1->var_off;









	true_32off = tnum_subreg(true_reg1->var_off);









	true_64off = true_reg1->var_off;









	uval = false_reg2->var_off.value;









	uval32 = (u32)tnum_subreg(false_reg2->var_off).value;









	sval = (s64)uval;









	sval32 = (s32)uval32;

















	switch (opcode) {









	/* JEQ/JNE comparison doesn't change the register equivalence.









	 *








@@ -14562,22 +14580,6 @@ static void reg_set_min_max(struct bpf_reg_state *true_reg1,







	}









}

















/* Same as above, but for the case that dst_reg holds a constant and src_reg is









 * the variable reg.









 */









static void reg_set_min_max_inv(struct bpf_reg_state *true_reg,









				struct bpf_reg_state *false_reg,









				u64 uval, u32 uval32,









				u8 opcode, bool is_jmp32)









{









	opcode = flip_opcode(opcode);









	/* This uses zero as "not present in table"; luckily the zero opcode,









	 * BPF_JA, can't get here.









	 */









	if (opcode)









		reg_set_min_max(true_reg, false_reg, uval, uval32, opcode, is_jmp32);









}

















/* Regs are known to be equal, so intersect their min/max/var_off */









static void __reg_combine_min_max(struct bpf_reg_state *src_reg,









				  struct bpf_reg_state *dst_reg)








@@ -14902,53 +14904,32 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,







		return -EFAULT;









	other_branch_regs = other_branch->frame[other_branch->curframe]->regs;

















	/* detect if we are comparing against a constant value so we can adjust









	 * our min/max values for our dst register.









	 * this is only legit if both are scalars (or pointers to the same









	 * object, I suppose, see the PTR_MAYBE_NULL related if block below),









	 * because otherwise the different base pointers mean the offsets aren't









	 * comparable.









	 */









	if (BPF_SRC(insn->code) == BPF_X) {









		struct bpf_reg_state *src_reg = &regs[insn->src_reg];









		reg_set_min_max(&other_branch_regs[insn->dst_reg],









				&other_branch_regs[insn->src_reg],









				dst_reg, src_reg, opcode, is_jmp32);

















		if (dst_reg->type == SCALAR_VALUE &&









		    src_reg->type == SCALAR_VALUE) {









			if (tnum_is_const(src_reg->var_off) ||









			    (is_jmp32 &&









			     tnum_is_const(tnum_subreg(src_reg->var_off))))









				reg_set_min_max(&other_branch_regs[insn->dst_reg],









						dst_reg,









						src_reg->var_off.value,









						tnum_subreg(src_reg->var_off).value,









						opcode, is_jmp32);









			else if (tnum_is_const(dst_reg->var_off) ||









				 (is_jmp32 &&









				  tnum_is_const(tnum_subreg(dst_reg->var_off))))









				reg_set_min_max_inv(&other_branch_regs[insn->src_reg],









						    src_reg,









						    dst_reg->var_off.value,









						    tnum_subreg(dst_reg->var_off).value,









						    opcode, is_jmp32);









			else if (!is_jmp32 &&









				 (opcode == BPF_JEQ || opcode == BPF_JNE))









		    src_reg->type == SCALAR_VALUE &&









		    !is_jmp32 && (opcode == BPF_JEQ || opcode == BPF_JNE)) {









			/* Comparing for equality, we can combine knowledge */









			reg_combine_min_max(&other_branch_regs[insn->src_reg],









					    &other_branch_regs[insn->dst_reg],









					    src_reg, dst_reg, opcode);









			if (src_reg->id &&









			    !WARN_ON_ONCE(src_reg->id != other_branch_regs[insn->src_reg].id)) {









				find_equal_scalars(this_branch, src_reg);









				find_equal_scalars(other_branch, &other_branch_regs[insn->src_reg]);









			}

















		}









	} else if (dst_reg->type == SCALAR_VALUE) {









	} else /* BPF_SRC(insn->code) == BPF_K */ {









		reg_set_min_max(&other_branch_regs[insn->dst_reg],









					dst_reg, insn->imm, (u32)insn->imm,









				src_reg /* fake one */,









				dst_reg, src_reg /* same fake one */,









				opcode, is_jmp32);









	}

















	if (BPF_SRC(insn->code) == BPF_X &&









	    src_reg->type == SCALAR_VALUE && src_reg->id &&









	    !WARN_ON_ONCE(src_reg->id != other_branch_regs[insn->src_reg].id)) {









		find_equal_scalars(this_branch, src_reg);









		find_equal_scalars(other_branch, &other_branch_regs[insn->src_reg]);









	}









	if (dst_reg->type == SCALAR_VALUE && dst_reg->id &&









	    !WARN_ON_ONCE(dst_reg->id != other_branch_regs[insn->dst_reg].id)) {









		find_equal_scalars(this_branch, dst_reg);