This is a note to let you know that I've just added the patch titled bpf: Do not zero-extend kfunc return values to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: bpf-do-not-zero-extend-kfunc-return-values.patch and it can be found in the queue-5.15 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit ff1fd5812085ab2d56c3e9a17fc5da73672a1f3c Author: Björn Töpel <bjorn@xxxxxxxxxxxx> Date: Wed Dec 7 11:35:40 2022 +0100 bpf: Do not zero-extend kfunc return values [ Upstream commit d35af0a7feb077c43ff0233bba5a8c6e75b73e35 ] In BPF all global functions, and BPF helpers return a 64-bit value. For kfunc calls, this is not the case, and they can return e.g. 32-bit values. The return register R0 for kfuncs calls can therefore be marked as subreg_def != DEF_NOT_SUBREG. In general, if a register is marked with subreg_def != DEF_NOT_SUBREG, some archs (where bpf_jit_needs_zext() returns true) require the verifier to insert explicit zero-extension instructions. For kfuncs calls, however, the caller should do sign/zero extension for return values. In other words, the compiler is responsible to insert proper instructions, not the verifier. An example, provided by Yonghong Song: $ cat t.c extern unsigned foo(void); unsigned bar1(void) { return foo(); } unsigned bar2(void) { if (foo()) return 10; else return 20; } $ clang -target bpf -mcpu=v3 -O2 -c t.c && llvm-objdump -d t.o t.o: file format elf64-bpf Disassembly of section .text: 0000000000000000 <bar1>: 0: 85 10 00 00 ff ff ff ff call -0x1 1: 95 00 00 00 00 00 00 00 exit 0000000000000010 <bar2>: 2: 85 10 00 00 ff ff ff ff call -0x1 3: bc 01 00 00 00 00 00 00 w1 = w0 4: b4 00 00 00 14 00 00 00 w0 = 0x14 5: 16 01 01 00 00 00 00 00 if w1 == 0x0 goto +0x1 <LBB1_2> 6: b4 00 00 00 0a 00 00 00 w0 = 0xa 0000000000000038 <LBB1_2>: 7: 95 00 00 00 00 00 00 00 exit If the return value of 'foo()' is used in the BPF program, the proper zero-extension will be done. Currently, the verifier correctly marks, say, a 32-bit return value as subreg_def != DEF_NOT_SUBREG, but will fail performing the actual zero-extension, due to a verifier bug in opt_subreg_zext_lo32_rnd_hi32(). load_reg is not properly set to R0, and the following path will be taken: if (WARN_ON(load_reg == -1)) { verbose(env, "verifier bug. zext_dst is set, but no reg is defined\n"); return -EFAULT; } A longer discussion from v1 can be found in the link below. Correct the verifier by avoiding doing explicit zero-extension of R0 for kfunc calls. Note that R0 will still be marked as a sub-register for return values smaller than 64-bit. Fixes: 83a2881903f3 ("bpf: Account for BPF_FETCH in insn_has_def32()") Link: https://lore.kernel.org/bpf/20221202103620.1915679-1-bjorn@xxxxxxxxxx/ Suggested-by: Yonghong Song <yhs@xxxxxxxx> Signed-off-by: Björn Töpel <bjorn@xxxxxxxxxxxx> Acked-by: Yonghong Song <yhs@xxxxxx> Link: https://lore.kernel.org/r/20221207103540.396496-1-bjorn@xxxxxxxxxx Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index b1ca4dbdeecf..488225bb42f6 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -12132,6 +12132,10 @@ static int opt_subreg_zext_lo32_rnd_hi32(struct bpf_verifier_env *env, if (!bpf_jit_needs_zext() && !is_cmpxchg_insn(&insn)) continue; + /* Zero-extension is done by the caller. */ + if (bpf_pseudo_kfunc_call(&insn)) + continue; + if (WARN_ON(load_reg == -1)) { verbose(env, "verifier bug. zext_dst is set, but no reg is defined\n"); return -EFAULT;