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Date: Tue, 7 Jul 2020 09:05:28 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Masahiro Yamada <masahiroy@...nel.org>,
Jakub Kicinski <kuba@...nel.org>
Cc: Will Deacon <will@...nel.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
"Paul E. McKenney" <paulmck@...nel.org>,
Kees Cook <keescook@...omium.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
clang-built-linux <clang-built-linux@...glegroups.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
linux-arch <linux-arch@...r.kernel.org>,
linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-pci@...r.kernel.org, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 00/22] add support for Clang LTO
On Tue, Jul 07, 2020 at 08:51:07AM -0700, Sami Tolvanen wrote:
> After spending some time debugging this with Nick, it looks like the
> error is caused by a recent optimization change in LLVM, which together
> with the inlining of ur_load_imm_any into jeq_imm, changes a runtime
> check in FIELD_FIT that would always fail, to a compile-time check that
> breaks the build. In jeq_imm, we have:
>
> /* struct bpf_insn: _s32 imm */
> u64 imm = insn->imm; /* sign extend */
> ...
> if (imm >> 32) { /* non-zero only if insn->imm is negative */
> /* inlined from ur_load_imm_any */
> u32 __imm = imm >> 32; /* therefore, always 0xffffffff */
>
> /*
> * __imm has a value known at compile-time, which means
> * __builtin_constant_p(__imm) is true and we end up with
> * essentially this in __BF_FIELD_CHECK:
> */
> if (__builtin_constant_p(__imm) && __imm <= 255)
Should be __imm > 255, of course, which means the compiler will generate
a call to __compiletime_assert.
> Jiong, Jakub, do you see any issues here?
(Jiong's email bounced, so removing from the recipient list.)
Sami
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