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s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc:content-transfer-encoding; bh=z8N6y2cPyajfBd+3nVitG0kgP25GzlYOsIPLOQdJGoc=; b=uHwkxgPa+Xb1T1/9QeCv9bBhsiOqlK3Z6gxxXV9be0NkcaBBvRURg7w/+Keu8qslIW buJSmDRQHrZqqJJVJYsByrGMWeyQjAHSGJy/Ot7DmES8pnL7ADJ9f5GB+q66zPlMFuM0 YkYIZ3W0K/ZSAnrhlNDMdPBnfyUyyNdHt7mKYx7cAihv9+sQNkiD0/MNDhRuYmyJG1jG MZ2iQkW2AKL85s+4f3P2apXpD6yZLBDxIidaZnPofHsYywc9LwrZNMseyJQ1YBEI0G0Z KyTgK606iECW/K1K0VY7xb9X9UYSpVAY5wG9dFub6rDxxlM4bIRaL3o/IKGG1zrjD3TD 86sA== X-Gm-Message-State: AOAM533lP4zsv8FBjMvodJjChWerK1Et71uwRFdsAitNAQvFoVKMFGj6 AUUngbQ624IzJHylxg83c5309n/nHej1pyghJj9a16EQwMKL X-Google-Smtp-Source: ABdhPJysMeU9/7fRt6JK4OFvId+se6U/bUzu7nWj/WCLpn2m/5WRXPoo/wwtjsa0wMRHGiAfoP0ywOfeRD7PkmHi7VI= X-Received: by 2002:a17:906:a413:: with SMTP id l19mr7167792ejz.421.1616197669914; Fri, 19 Mar 2021 16:47:49 -0700 (PDT) MIME-Version: 1.0 References: <20210317232657.mdnsuoqx6nbddjgt@google.com> In-Reply-To: From: Bill Wendling Date: Fri, 19 Mar 2021 16:47:38 -0700 Message-ID: Subject: Re: pahole -J usage in kernel scripts/link-vmlinux.sh To: Andrii Nakryiko Cc: Fangrui Song , dwarves@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: dwarves@vger.kernel.org On Fri, Mar 19, 2021 at 4:38 PM Andrii Nakryiko wrote: > > On Fri, Mar 19, 2021 at 4:30 PM Bill Wendling wrote: > > > > On Fri, Mar 19, 2021 at 3:44 PM Andrii Nakryiko > > wrote: > > > > > > On Wed, Mar 17, 2021 at 4:28 PM Fangrui Song wro= te: > > > > > > > > Hi BTF folks, > > > > > > > > I have discovered some problems with pahole -J. > > > > Its usage in kernel scripts/link-vmlinux.sh is like (make LLVM=3D1 = bzImage): > > > > > > > > ld.lld -m elf_x86_64 --emit-relocs --discard-none -z max-page-size= =3D0x200000 --build-id=3Dsha1 --orphan-handling=3Dwarn -o .tmp_vmlinux.btf = -T ./arch/x86/kernel/vmlinux.lds --whole-archive arch/x86/kernel/head_64.o = arch/x86/kernel/head64.o arch/x86/kernel/ebda.o arch/x86/kernel/platform-qu= irks.o init/built-in.a usr/built-in.a arch/x86/built-in.a kernel/built-in.a= certs/built-in.a mm/built-in.a fs/built-in.a ipc/built-in.a security/built= -in.a crypto/built-in.a block/built-in.a lib/built-in.a arch/x86/lib/built-= in.a lib/lib.a arch/x86/lib/lib.a drivers/built-in.a sound/built-in.a net/b= uilt-in.a virt/built-in.a arch/x86/pci/built-in.a arch/x86/power/built-in.a= arch/x86/video/built-in.a --no-whole-archive --start-group --end-group > > > > pahole -J .tmp_vmlinux.btf > > > > llvm-objcopy --only-section=3D.BTF --set-section-flags .BTF=3Dalloc= ,readonly --strip-all .tmp_vmlinux.btf .btf.vmlinux.bin.o > > > > > > > > pahole -J adds .BTF and rewrites .tmp_vmlinux.btf, then llvm-objcop= y produces .btf.vmlinux.bin.o of just one section. > > > > Why doesn't pahole provide a command generating an object file with= just the .BTF section? > > > > > > > > > > We just recently discussed adding this. So the reason is that > > > historically pahole never had this feature and no one bothered to add > > > it. > > > > > > > > > > > > > > > When I contributed https://git.kernel.org/linus/90ceddcb495008ac8ba= 7a3dce297841efcd7d584 , > > > > I remember pahole at that time added a non-SHF_ALLOC .BTF , now (1.= 20) .BTF becomes SHF_ALLOC. > > > > > > I don't think anything changed in 1.20 about how .BTF is added. pahol= e > > > still uses the same llvm-objcopt and it doesn't add SHF_ALLOC. And I > > > just double-checked that after running pahole -j .tmp_vmlinux.btf has > > > non-allocatable .BTF section: > > > > > > $ llvm-readelf -S ~/tmp/pahole-vmlinux-output.o | grep BTF > > > [13] .BTF_ids PROGBITS ffffffff822b6804 14b6804 > > > 000510 00 A 0 0 1 > > > [43] .BTF PROGBITS 0000000000000000 18bc0e8c > > > 4c8056 00 0 0 1 > > > > > > > > > > This is problematic if pahole does not have full-fledged binary man= ipulation ability (objcopy,llvm-objcopy). > > > > > > > > In particular, there are two bugs: > > > > > > > > * pahole does not respect max-page-size (p_align of PT_LOAD). See t= he .text section, its > > > > sh_offset !=3D sh_addr (mod max-page-size) > > > > > > > > Section Headers: > > > > [Nr] Name Type Address Off = Size ES Flg Lk Inf Al > > > > [ 0] NULL 0000000000000000 000000 = 000000 00 0 0 0 > > > > - [ 1] .text PROGBITS ffffffff81000000 200000 1= 003917 00 AX 0 0 4096 > > > > + [ 1] .text PROGBITS ffffffff81000000 001000 e= 0169a 00 AX 0 0 4096 > > > > > > > > * pahole does not rewrite p_offset/p_filesz of PT_LOAD segments. > > > > Because of the first bug, pahole -J rewritten object file genera= lly has small offsets. > > > > If p_offset/p_filesz of PT_LOAD segments are not rewritten, the = file offset range of .symtab may be within > > > > a PT_LOAD range. llvm-objcopy --strip-all considers .symtab as p= art of the PT_LOAD and refuses --strip-all: > > > > > > > > error: '.tmp_vmlinux.btf': string table '.strtab' cannot be remo= ved because it is referenced by the symbol table '.symtab' > > > > > > > > This is very rare, though. > > > > > > > > > > > > So I suggest: > > > > > > > > * pahole -J: restore the previous non-SHF_ALLOC behavior. Don't rew= rite sh_offset of existing sections. > > > > > > so nothing changed (at least as of 1.20) about how pahole adds .BTF, > > > so I'd like to understand why our observations differ > > > > > Here's one things we're seeing (note, we're using a kernel based on > > 4.15). Before we run pahole, we have this. Notice the offset of the > > '.text' section: > > > > [ 0] NULL 0000000000000000 000000 > > 000000 00 0 0 0 > > [ 1] .text PROGBITS ffffffff81000000 200000 > > e0169a 00 AX 0 0 4096 > > [ 2] .notes NOTE ffffffff81e0169c 100169c > > 000024 00 A 0 0 4 > > ... > > [24] .BTF PROGBITS ffffffff825caf40 17caf40 > > 000000 00 WA 0 0 1 > > [25] .BTF_ids PROGBITS ffffffff825caf40 17caf40 > > 0004c0 00 A 0 0 1 > > > > After running pahole, we get this: > > > > [ 0] NULL 0000000000000000 000000 > > 000000 00 0 0 0 > > [ 1] .text PROGBITS ffffffff81000000 001000 > > e0169a 00 AX 0 0 4096 > > [ 2] .notes NOTE ffffffff81e0169c e0269c > > 000024 00 A 0 0 4 > > ... > > [24] .BTF PROGBITS ffffffff825caf40 11ecf40 > > 3c1ebe 00 WA 0 0 1 > > [25] .BTF_ids PROGBITS ffffffff825caf40 15aedfe > > 0004c0 00 A 0 0 1 > > > > The offset of '.text' changed. This is because `elf_update` decides > > what the offsets should be. Did the linker scripts change between 4.15 > > and top-of-tree to mark .BTF as non-allocatable? > > maybe so, but .tmp_vmlinux.btf which gets .BTF is discarded and not > used anymore. We only dump .BTF section into a separate ELF, which is > linked into the final vmlinux image as the next step. So pahole > doesn't rewrite the final binary. > Right. The problem is with the follow-up command: ${OBJCOPY} --only-section=3D.BTF --set-section-flags .BTF=3Dalloc,r= eadonly \ --strip-all ${1} ${2} 2>/dev/null The file .tmp_vmlinux.btf isn't "correct" (see maskray's explanation), so this command fails to write a valid ELF file. -bw