Re: linux-next: build warning after merge of the bpf-next tree
On 1/14/20 6:23 AM, Alexei Starovoitov wrote: On Sun, Jan 12, 2020 at 8:33 PM Zong Li wrote: I'm not quite familiar with btf, so I have no idea why there are two weak symbols be added in 8580ac9404f6 ("bpf: Process in-kernel BTF") I can explain what these weak symbols are for, but that won't change the fact that compiler or linker are buggy. The weak symbols should work in all cases and compiler should pick correct relocation. In this case it sounds that compiler picked relative relocation and failed to reach zero from that address. Sorry for the response delay: I now agree that there is nothing weird about those relocations. All compiler/linker I took a look at (arm64, ppc64 and riscv64) correctly emit an absolute relocation to the address 0 in case of a weak unresolved symbol, so there's no buggy compiler/linker. And regarding ppc warning, the kernel being compiled as -pie, the scripts looks for absolute relocations which it considers as "bad", except for one that is known to be weak and that is ignored: I have just sent a patch to fix this script so that weak undefined symbol relocations are not considered as bad. Thanks, Alex
Re: linux-next: build warning after merge of the bpf-next tree
On Sun, Jan 12, 2020 at 8:33 PM Zong Li wrote: > > I'm not quite familiar with btf, so I have no idea why there are two > weak symbols be added in 8580ac9404f6 ("bpf: Process in-kernel BTF") I can explain what these weak symbols are for, but that won't change the fact that compiler or linker are buggy. The weak symbols should work in all cases and compiler should pick correct relocation. In this case it sounds that compiler picked relative relocation and failed to reach zero from that address.
Re: linux-next: build warning after merge of the bpf-next tree
On Sat, Jan 11, 2020 at 10:31 PM Alexandre Ghiti wrote: > > > On 1/10/20 7:20 PM, Palmer Dabbelt wrote: > > On Fri, 10 Jan 2020 14:28:17 PST (-0800), alexan...@ghiti.fr wrote: > >> Hi guys, > >> > >> On 10/27/19 8:02 PM, Stephen Rothwell wrote: > >>> Hi all, > >>> > >>> On Fri, 18 Oct 2019 10:56:57 +1100 Stephen Rothwell > >>> wrote: > Hi all, > > After merging the bpf-next tree, today's linux-next build (powerpc > ppc64_defconfig) produced this warning: > > WARNING: 2 bad relocations > c1998a48 R_PPC64_ADDR64 _binary__btf_vmlinux_bin_start > c1998a50 R_PPC64_ADDR64 _binary__btf_vmlinux_bin_end > > Introduced by commit > > 8580ac9404f6 ("bpf: Process in-kernel BTF") > >>> This warning now appears in the net-next tree build. > >>> > >>> > >> I bump that thread up because Zong also noticed that 2 new > >> relocations for > >> those symbols appeared in my riscv relocatable kernel branch following > >> that commit. > >> > >> I also noticed 2 new relocations R_AARCH64_ABS64 appearing in arm64 > >> kernel. > >> > >> Those 2 weak undefined symbols have existed since commit > >> 341dfcf8d78e ("btf: expose BTF info through sysfs") but this is the fact > >> to declare those symbols into btf.c that produced those relocations. > >> > >> I'm not sure what this all means, but this is not something I expected > >> for riscv for > >> a kernel linked with -shared/-fpie. Maybe should we just leave them to > >> zero ? > >> > >> I think that deserves a deeper look if someone understands all this > >> better than I do. > > > > Can you give me a pointer to your tree and how to build a relocatable > > kernel? > > Weak undefined symbols have the absolute value 0, > > > So according to you the 2 new relocations R_RISCV_64 are normal and > should not > be modified at runtime right ? > > > > but the kernel is linked at > > an address such that 0 can't be reached by normal means. When I added > > support > > to binutils for this I did it in a way that required almost no code -- > > essetially I just stopped dissallowing x0 as a possible base register > > for PCREL > > relocations, which results in 0 always being accessible. I just > > wanted to get > > the kernel to build again, so I didn't worry about chasing around all the > > addressing modes. The PIC/PIE support generates different relocations > > and I > > wouldn't be surprised if I just missed one (or more likely all) of them. > > > > It's probably a simple fix, though I feel like every time I say that > > about the > > linker I end up spending a month in there... > > You can find it here: > > https://github.com/AlexGhiti/riscv-linux/tree/int/alex/riscv_relocatable_v1 > > Zong fixed the bug introduced by those 2 new relocations and everything > works > like a charm, so I'm not sure you have to dig in the linker :) > I'm not quite familiar with btf, so I have no idea why there are two weak symbols be added in 8580ac9404f6 ("bpf: Process in-kernel BTF") as well, According on relocation mechanism, maybe it is unnecessary to handle weak undefined symbol at this time, because there is no substantive help to relocate the absolute value 0. I just simply ignore the non-relative relocation types to make processing can go forward, and it works for me based on v5.5-rc5. > Alex >
Re: linux-next: build warning after merge of the bpf-next tree
On 1/10/20 7:20 PM, Palmer Dabbelt wrote: On Fri, 10 Jan 2020 14:28:17 PST (-0800), alexan...@ghiti.fr wrote: Hi guys, On 10/27/19 8:02 PM, Stephen Rothwell wrote: Hi all, On Fri, 18 Oct 2019 10:56:57 +1100 Stephen Rothwell wrote: Hi all, After merging the bpf-next tree, today's linux-next build (powerpc ppc64_defconfig) produced this warning: WARNING: 2 bad relocations c1998a48 R_PPC64_ADDR64 _binary__btf_vmlinux_bin_start c1998a50 R_PPC64_ADDR64 _binary__btf_vmlinux_bin_end Introduced by commit 8580ac9404f6 ("bpf: Process in-kernel BTF") This warning now appears in the net-next tree build. I bump that thread up because Zong also noticed that 2 new relocations for those symbols appeared in my riscv relocatable kernel branch following that commit. I also noticed 2 new relocations R_AARCH64_ABS64 appearing in arm64 kernel. Those 2 weak undefined symbols have existed since commit 341dfcf8d78e ("btf: expose BTF info through sysfs") but this is the fact to declare those symbols into btf.c that produced those relocations. I'm not sure what this all means, but this is not something I expected for riscv for a kernel linked with -shared/-fpie. Maybe should we just leave them to zero ? I think that deserves a deeper look if someone understands all this better than I do. Can you give me a pointer to your tree and how to build a relocatable kernel? Weak undefined symbols have the absolute value 0, So according to you the 2 new relocations R_RISCV_64 are normal and should not be modified at runtime right ? but the kernel is linked at an address such that 0 can't be reached by normal means. When I added support to binutils for this I did it in a way that required almost no code -- essetially I just stopped dissallowing x0 as a possible base register for PCREL relocations, which results in 0 always being accessible. I just wanted to get the kernel to build again, so I didn't worry about chasing around all the addressing modes. The PIC/PIE support generates different relocations and I wouldn't be surprised if I just missed one (or more likely all) of them. It's probably a simple fix, though I feel like every time I say that about the linker I end up spending a month in there... You can find it here: https://github.com/AlexGhiti/riscv-linux/tree/int/alex/riscv_relocatable_v1 Zong fixed the bug introduced by those 2 new relocations and everything works like a charm, so I'm not sure you have to dig in the linker :) Alex
Re: linux-next: build warning after merge of the bpf-next tree
On 1/10/20 6:18 PM, Alexei Starovoitov wrote: On Fri, Jan 10, 2020 at 2:28 PM Alexandre Ghiti wrote: Hi guys, On 10/27/19 8:02 PM, Stephen Rothwell wrote: Hi all, On Fri, 18 Oct 2019 10:56:57 +1100 Stephen Rothwell wrote: Hi all, After merging the bpf-next tree, today's linux-next build (powerpc ppc64_defconfig) produced this warning: WARNING: 2 bad relocations c1998a48 R_PPC64_ADDR64_binary__btf_vmlinux_bin_start c1998a50 R_PPC64_ADDR64_binary__btf_vmlinux_bin_end Introduced by commit 8580ac9404f6 ("bpf: Process in-kernel BTF") This warning now appears in the net-next tree build. I bump that thread up because Zong also noticed that 2 new relocations for those symbols appeared in my riscv relocatable kernel branch following that commit. I also noticed 2 new relocations R_AARCH64_ABS64 appearing in arm64 kernel. Those 2 weak undefined symbols have existed since commit 341dfcf8d78e ("btf: expose BTF info through sysfs") but this is the fact to declare those symbols into btf.c that produced those relocations. I'm not sure what this all means, but this is not something I expected for riscv for a kernel linked with -shared/-fpie. Maybe should we just leave them to zero ? I think that deserves a deeper look if someone understands all this better than I do. Are you saying there is a warning for arm64 as well? Nop. Can ppc folks explain the above warning? What does it mean "2 bad relocations"? This is what I'd like to understand too, it is not clear in the ppc tool that outputs this message why it is considered 'bad'. The code is doing: extern char __weak _binary__btf_vmlinux_bin_start[]; extern char __weak _binary__btf_vmlinux_bin_end[]; Since they are weak they should be zero when not defined. What's the issue? There likely is no issue, I just want to make sure those relocations are legitimate and I want to understand what we should do with those. At the moment arm64 does not relocate those at runtime and purely ignore them: is this the right thing to do ?
Re: Re: linux-next: build warning after merge of the bpf-next tree
On Fri, 10 Jan 2020 14:28:17 PST (-0800), alexan...@ghiti.fr wrote: Hi guys, On 10/27/19 8:02 PM, Stephen Rothwell wrote: Hi all, On Fri, 18 Oct 2019 10:56:57 +1100 Stephen Rothwell wrote: Hi all, After merging the bpf-next tree, today's linux-next build (powerpc ppc64_defconfig) produced this warning: WARNING: 2 bad relocations c1998a48 R_PPC64_ADDR64_binary__btf_vmlinux_bin_start c1998a50 R_PPC64_ADDR64_binary__btf_vmlinux_bin_end Introduced by commit 8580ac9404f6 ("bpf: Process in-kernel BTF") This warning now appears in the net-next tree build. I bump that thread up because Zong also noticed that 2 new relocations for those symbols appeared in my riscv relocatable kernel branch following that commit. I also noticed 2 new relocations R_AARCH64_ABS64 appearing in arm64 kernel. Those 2 weak undefined symbols have existed since commit 341dfcf8d78e ("btf: expose BTF info through sysfs") but this is the fact to declare those symbols into btf.c that produced those relocations. I'm not sure what this all means, but this is not something I expected for riscv for a kernel linked with -shared/-fpie. Maybe should we just leave them to zero ? I think that deserves a deeper look if someone understands all this better than I do. Can you give me a pointer to your tree and how to build a relocatable kernel? Weak undefined symbols have the absolute value 0, but the kernel is linked at an address such that 0 can't be reached by normal means. When I added support to binutils for this I did it in a way that required almost no code -- essetially I just stopped dissallowing x0 as a possible base register for PCREL relocations, which results in 0 always being accessible. I just wanted to get the kernel to build again, so I didn't worry about chasing around all the addressing modes. The PIC/PIE support generates different relocations and I wouldn't be surprised if I just missed one (or more likely all) of them. It's probably a simple fix, though I feel like every time I say that about the linker I end up spending a month in there...
Re: Re: linux-next: build warning after merge of the bpf-next tree
On Fri, Jan 10, 2020 at 2:28 PM Alexandre Ghiti wrote: > > Hi guys, > > On 10/27/19 8:02 PM, Stephen Rothwell wrote: > > Hi all, > > > > On Fri, 18 Oct 2019 10:56:57 +1100 Stephen Rothwell > > wrote: > >> Hi all, > >> > >> After merging the bpf-next tree, today's linux-next build (powerpc > >> ppc64_defconfig) produced this warning: > >> > >> WARNING: 2 bad relocations > >> c1998a48 R_PPC64_ADDR64_binary__btf_vmlinux_bin_start > >> c1998a50 R_PPC64_ADDR64_binary__btf_vmlinux_bin_end > >> > >> Introduced by commit > >> > >>8580ac9404f6 ("bpf: Process in-kernel BTF") > > This warning now appears in the net-next tree build. > > > > > I bump that thread up because Zong also noticed that 2 new relocations for > those symbols appeared in my riscv relocatable kernel branch following > that commit. > > I also noticed 2 new relocations R_AARCH64_ABS64 appearing in arm64 kernel. > > Those 2 weak undefined symbols have existed since commit > 341dfcf8d78e ("btf: expose BTF info through sysfs") but this is the fact > to declare those symbols into btf.c that produced those relocations. > > I'm not sure what this all means, but this is not something I expected > for riscv for > a kernel linked with -shared/-fpie. Maybe should we just leave them to > zero ? > > I think that deserves a deeper look if someone understands all this > better than I do. Are you saying there is a warning for arm64 as well? Can ppc folks explain the above warning? What does it mean "2 bad relocations"? The code is doing: extern char __weak _binary__btf_vmlinux_bin_start[]; extern char __weak _binary__btf_vmlinux_bin_end[]; Since they are weak they should be zero when not defined. What's the issue?
Re: Re: linux-next: build warning after merge of the bpf-next tree
Hi guys, On 10/27/19 8:02 PM, Stephen Rothwell wrote: Hi all, On Fri, 18 Oct 2019 10:56:57 +1100 Stephen Rothwell wrote: Hi all, After merging the bpf-next tree, today's linux-next build (powerpc ppc64_defconfig) produced this warning: WARNING: 2 bad relocations c1998a48 R_PPC64_ADDR64_binary__btf_vmlinux_bin_start c1998a50 R_PPC64_ADDR64_binary__btf_vmlinux_bin_end Introduced by commit 8580ac9404f6 ("bpf: Process in-kernel BTF") This warning now appears in the net-next tree build. I bump that thread up because Zong also noticed that 2 new relocations for those symbols appeared in my riscv relocatable kernel branch following that commit. I also noticed 2 new relocations R_AARCH64_ABS64 appearing in arm64 kernel. Those 2 weak undefined symbols have existed since commit 341dfcf8d78e ("btf: expose BTF info through sysfs") but this is the fact to declare those symbols into btf.c that produced those relocations. I'm not sure what this all means, but this is not something I expected for riscv for a kernel linked with -shared/-fpie. Maybe should we just leave them to zero ? I think that deserves a deeper look if someone understands all this better than I do. Alex
Re: linux-next: build warning after merge of the bpf-next tree
Hi all, On Fri, 18 Oct 2019 10:56:57 +1100 Stephen Rothwell wrote: > > Hi all, > > After merging the bpf-next tree, today's linux-next build (powerpc > ppc64_defconfig) produced this warning: > > WARNING: 2 bad relocations > c1998a48 R_PPC64_ADDR64_binary__btf_vmlinux_bin_start > c1998a50 R_PPC64_ADDR64_binary__btf_vmlinux_bin_end > > Introduced by commit > > 8580ac9404f6 ("bpf: Process in-kernel BTF") This warning now appears in the net-next tree build. -- Cheers, Stephen Rothwell pgpI1m7wrJHG7.pgp Description: OpenPGP digital signature
Re: linux-next: build warning after merge of the bpf-next tree
On Fri, Oct 18, 2019 at 10:56:57AM +1100, Stephen Rothwell wrote: > Hi all, > > After merging the bpf-next tree, today's linux-next build (powerpc > ppc64_defconfig) produced this warning: > > WARNING: 2 bad relocations > c1998a48 R_PPC64_ADDR64_binary__btf_vmlinux_bin_start > c1998a50 R_PPC64_ADDR64_binary__btf_vmlinux_bin_end Can ppc folks help me figure out what this warning means?
linux-next: build warning after merge of the bpf-next tree
Hi all, After merging the bpf-next tree, today's linux-next build (powerpc ppc64_defconfig) produced this warning: WARNING: 2 bad relocations c1998a48 R_PPC64_ADDR64_binary__btf_vmlinux_bin_start c1998a50 R_PPC64_ADDR64_binary__btf_vmlinux_bin_end Introduced by commit 8580ac9404f6 ("bpf: Process in-kernel BTF") -- Cheers, Stephen Rothwell pgpHWYLSLLFzL.pgp Description: OpenPGP digital signature