From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-lf1-f52.google.com (mail-lf1-f52.google.com [209.85.167.52]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3F24C4C6C for ; Mon, 22 Aug 2022 23:55:30 +0000 (UTC) Received: by mail-lf1-f52.google.com with SMTP id bt10so4479849lfb.1 for ; Mon, 22 Aug 2022 16:55:30 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc; bh=niLyS67g7armALDit3lWWwA/6MSSHSl2h7o2NXDxzOY=; b=Ab/JGunfBkj09p8UYHG+04gv816Wj6N7QlDDVVmzM4DFI5gLIqcVRpnuYpYDw4zN9Z GC9DTAq+2w/DGPakMHbgeqIEa04A3tzy5mxAo2FPHW9PtLJK5/LHo73RxYC61jrvd9tD H/zdqD6e5RJqffQ4sqcDj7ewWi1otnmiDVBH/mQClKCeYi/oQnWx5RhAn+Y7IoQbgtNQ 2fM7FGnz3W4Jcxkcj0tbA39mtRCU9YQ+2J/Qh646fdCl3xiMG+XJUu80H6qsHSXu5FZu +YrxVK8USlIJxZ95dKRbjR0g42EEIlmTQkvNsmDimAbDB6uNwAfyIS5mrzzQhuR3Dc83 QOhA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc; bh=niLyS67g7armALDit3lWWwA/6MSSHSl2h7o2NXDxzOY=; b=6xAiScCMxiQ/7Ch34veUff5VjaYKV4+oNedUDJ/g04PWIOmoRJZ2DjvcuDrbDn2vjm Q31Tfg3UDKrw64hk23RYeKU4jN0ADRRpOs4d7Ltg4FSqvgvMzmP9DFJLtT/oHOMfTRDR YNVgVtB3Jdn9XAf275urQtL2gpWOGeDLjDwW8FwZXWDktSo3TolAoPKTcXmvYi/qaxke /g+s7l+aUGrt7trxxOvwJni9Uxu65o5auzlIiB8rTQmdaVsGzPqYu9yWL4GvjT257nV6 xFx4PE218dPnbWjHJWzpJ4chJos9fpyYYKTI3XxMSSDzYd8jl5H4nKUT7Ls+wxYXMo7B 2zCQ== X-Gm-Message-State: ACgBeo1OLziu1Toqtkxb58BCpNTt1EkifHv9reFFHxxZgmg9WQ+2/B6d YY6Cxk9X1PF3ymsAkU76o+EAapbeI8j5oUlWZ7yBZw== X-Google-Smtp-Source: AA6agR5apkHpwR0VwjGEEi6Q1l+gdgRdLUzQTlqNRG4Scr/20JoVUR8iHBbZdYKS7e/RZBvAA1BDCYIuwOH1X5uEI6o= X-Received: by 2002:a05:6512:29c:b0:492:db8e:5e77 with SMTP id j28-20020a056512029c00b00492db8e5e77mr3555610lfp.403.1661212528009; Mon, 22 Aug 2022 16:55:28 -0700 (PDT) Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20220805154231.31257-1-ojeda@kernel.org> <20220805154231.31257-10-ojeda@kernel.org> In-Reply-To: <20220805154231.31257-10-ojeda@kernel.org> From: Nick Desaulniers Date: Mon, 22 Aug 2022 16:55:15 -0700 Message-ID: Subject: Re: [PATCH v9 09/27] rust: add `compiler_builtins` crate To: Miguel Ojeda Cc: Linus Torvalds , Greg Kroah-Hartman , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, patches@lists.linux.dev, Jarkko Sakkinen , Alex Gaynor , Wedson Almeida Filho , Sven Van Asbroeck , Gary Guo , Boqun Feng , =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= , Masahiro Yamada Content-Type: text/plain; charset="UTF-8" On Fri, Aug 5, 2022 at 8:44 AM Miguel Ojeda wrote: > > Rust provides `compiler_builtins` as a port of LLVM's `compiler-rt`. > Since we do not need the vast majority of them, we avoid the > dependency by providing our own crate. > > Co-developed-by: Alex Gaynor > Signed-off-by: Alex Gaynor > Co-developed-by: Wedson Almeida Filho > Signed-off-by: Wedson Almeida Filho > Co-developed-by: Sven Van Asbroeck > Signed-off-by: Sven Van Asbroeck > Co-developed-by: Gary Guo > Signed-off-by: Gary Guo > Signed-off-by: Miguel Ojeda I still really really do not like this patch. Thoughts below. > --- > rust/compiler_builtins.rs | 63 +++++++++++++++++++++++++++++++++++++++ > 1 file changed, 63 insertions(+) > create mode 100644 rust/compiler_builtins.rs > > diff --git a/rust/compiler_builtins.rs b/rust/compiler_builtins.rs > new file mode 100644 > index 000000000000..f8f39a3e6855 > --- /dev/null > +++ b/rust/compiler_builtins.rs > @@ -0,0 +1,63 @@ > +// SPDX-License-Identifier: GPL-2.0 > + > +//! Our own `compiler_builtins`. > +//! > +//! Rust provides [`compiler_builtins`] as a port of LLVM's [`compiler-rt`]. > +//! Since we do not need the vast majority of them, we avoid the dependency > +//! by providing this file. > +//! > +//! At the moment, some builtins are required that should not be. For instance, > +//! [`core`] has 128-bit integers functionality which we should not be compiling It's not just 128b, these are for double word sizes, so for 32b targets (I recall an earlier version of series supporting armv6) these symbols are for 64b types. > +//! in. We will work with upstream [`core`] to provide feature flags to disable > +//! the parts we do not need. For the moment, we define them to [`panic!`] at Where is the bug filed upstream (or in your issue tracker)? > +//! runtime for simplicity to catch mistakes, instead of performing surgery > +//! on `core.o`. Wait, so Kbuild performs surgery on alloc, but core is off limits? bah Core is huge! $ du -h rust/core.o 1.3M rust/core.o > +//! > +//! In any case, all these symbols are weakened to ensure we do not override > +//! those that may be provided by the rest of the kernel. > +//! > +//! [`compiler_builtins`]: https://github.com/rust-lang/compiler-builtins > +//! [`compiler-rt`]: https://compiler-rt.llvm.org/ > + > +#![feature(compiler_builtins)] > +#![compiler_builtins] > +#![no_builtins] > +#![no_std] > + > +macro_rules! define_panicking_intrinsics( > + ($reason: tt, { $($ident: ident, )* }) => { > + $( > + #[doc(hidden)] > + #[no_mangle] > + pub extern "C" fn $ident() { > + panic!($reason); > + } > + )* > + } > +); > + > +define_panicking_intrinsics!("`f32` should not be used", { > + __eqsf2, I don't see ^ this symbol in core. $ llvm-nm rust/core.o | grep "U __" | sort | tr -s " " U __gesf2 U __lesf2 U __udivti3 U __umodti3 U __unorddf2 U __unordsf2 U __x86_indirect_thunk_r11 so a few of these seem extraneous. Are you sure they're coming from core? (or was it the different arch support that was removed from v8 to facilitate v9?) __gesf2, __lesf2, and __unordsf2 all seem to be coming from <::classify> and <::to_bits::ct_f32_to_u32>. Surely we can avoid more f32 support by relying on objcopy -N/--strip-symbol=/--strip-symbols=filename to slice out/drop symbols from core.o? uses __umodti3+__udivti3 __unorddf2 is referenced by <::classify> and <::to_bits::ct_f64_to_u64>. Can we slice those 5 symbols out from core, or are they further referenced from within core? If we can, we can drop this patch outright, and avoid a false-negative when C code generates references to these symbols from the compiler runtime, for architectures where don't want to provide a full compiler runtime. (--rtlib=none is a pipe dream; first we need to get to the point where we're not -ffreestanding for all targets...) I suspect someone can iteratively whittle down core to avoid these symbols? ``` diff --git a/rust/Makefile b/rust/Makefile index 7700d3853404..e64a82c21156 100644 --- a/rust/Makefile +++ b/rust/Makefile @@ -359,6 +359,7 @@ $(obj)/core.o: $(RUST_LIB_SRC)/core/src/lib.rs $(obj)/target.json FORCE $(obj)/compiler_builtins.o: private rustc_objcopy = -w -W '__*' $(obj)/compiler_builtins.o: $(src)/compiler_builtins.rs $(obj)/core.o FORCE $(call if_changed_dep,rustc_library) + @$(OBJCOPY) --strip-symbols=$(obj)/strip-symbols.txt $(obj)/core.o $(obj)/alloc.o: private skip_clippy = 1 $(obj)/alloc.o: private skip_flags = -Dunreachable_pub ``` then define symbols in rust/strip-symbols.txt? I'm getting the sense that no, there are many functions like ::fmt ::fmt ::fmt that call many of these functions...so if you call format! on an i64/u64/usize you'll potentially panic the kernel? Seems kinda dangerous. I'm also seeing disassemblers having trouble completely disassembling core.o ... the more I look to see who's calling what, the more instances I find... > + __gesf2, > + __lesf2, > + __nesf2, > + __unordsf2, > +}); > + > +define_panicking_intrinsics!("`f64` should not be used", { > + __unorddf2, > +}); > + > +define_panicking_intrinsics!("`i128` should not be used", { > + __ashrti3, > + __muloti4, > + __multi3, > +}); > + > +define_panicking_intrinsics!("`u128` should not be used", { > + __ashlti3, > + __lshrti3, > + __udivmodti4, > + __udivti3, > + __umodti3, > +}); > -- > 2.37.1 > -- Thanks, ~Nick Desaulniers