From 45159b27637b0fef6d5ddb86fc7c46b13c77960f Mon Sep 17 00:00:00 2001 From: Ilya Leoshkevich Date: Fri, 12 Feb 2021 05:04:08 +0100 Subject: [PATCH] bpf: Clear subreg_def for global function return values test_global_func4 fails on s390 as reported by Yauheni in [1]. The immediate problem is that the zext code includes the instruction, whose result needs to be zero-extended, into the zero-extension patchlet, and if this instruction happens to be a branch, then its delta is not adjusted. As a result, the verifier rejects the program later. However, according to [2], as far as the verifier's algorithm is concerned and as specified by the insn_no_def() function, branching insns do not define anything. This includes call insns, even though one might argue that they define %r0. This means that the real problem is that zero extension kicks in at all. This happens because clear_caller_saved_regs() sets BPF_REG_0's subreg_def after global function calls. This can be fixed in many ways; this patch mimics what helper function call handling already does. [1] https://lore.kernel.org/bpf/20200903140542.156624-1-yauheni.kaliuta@redhat.com/ [2] https://lore.kernel.org/bpf/CAADnVQ+2RPKcftZw8d+B1UwB35cpBhpF5u3OocNh90D9pETPwg@mail.gmail.com/ Fixes: 51c39bb1d5d1 ("bpf: Introduce function-by-function verification") Reported-by: Yauheni Kaliuta Signed-off-by: Ilya Leoshkevich Signed-off-by: Daniel Borkmann Link: https://lore.kernel.org/bpf/20210212040408.90109-1-iii@linux.ibm.com --- kernel/bpf/verifier.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 11a242932a2c..16ba43352a5f 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -5279,8 +5279,9 @@ static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn, subprog); clear_caller_saved_regs(env, caller->regs); - /* All global functions return SCALAR_VALUE */ + /* All global functions return a 64-bit SCALAR_VALUE */ mark_reg_unknown(env, caller->regs, BPF_REG_0); + caller->regs[BPF_REG_0].subreg_def = DEF_NOT_SUBREG; /* continue with next insn after call */ return 0;