Re: [PATCH dwarves] btf_encoder: fix and complete filtering out zero-sized per-CPU variables

["Michal Suchánek" <msuchanek@suse.de>]

On Thu, May 27, 2021 at 01:47:41PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Thu, May 27, 2021 at 12:38:38PM -0300, Arnaldo Carvalho de Melo escreveu:
> > Em Thu, May 27, 2021 at 05:27:58PM +0200, Michal Suchánek escreveu:
> > > Hello,
> > > 
> > > On Thu, May 27, 2021 at 11:58:24AM -0300, Arnaldo Carvalho de Melo wrote:
> > > > Em Thu, May 27, 2021 at 11:55:10AM -0300, Arnaldo Carvalho de Melo escreveu:
> > > > > Em Mon, May 24, 2021 at 04:42:22PM -0700, Andrii Nakryiko escreveu:
> > > > > > btf_encoder is ignoring zero-sized per-CPU ELF symbols, but the same has to be
> > > > > > done for DWARF variables when matching them with ELF symbols. This is due to
> > > > > > zero-sized DWARF variables matching unrelated (non-zero-sized) variable that
> > > > > > happens to be allocated at the exact same address, leading to a lot of
> > > > > > confusion in BTF.
> > > > >  
> > > > > > See [0] for when this causes big problems.
> > > > >  
> > > > > >   [0] https://lore.kernel.org/bpf/CAEf4BzZ0-sihSL-UAm21JcaCCY92CqfNxycHRZYXcoj8OYb=wA@mail.gmail.com/
> > > > 
> > > > I also added this:
> > > > 
> > > > Reported-by: Michal Suchánek <msuchanek@suse.de>
> > > > 
> > > > Michal, so you tested this patch and verified it fixed the problem? If
> > > > so please let me know so that I also add:
> > > 
> > > This is the first time I see this patch.
> > > 
> > > Given that linux-next does not build for me at the moment
> > > I don't think I will test it soon.
> > 
> > Ok, I'm test building with torvalds/master, will try with linux-next
> > afterwards,
> 
> I build and booted torvalds/master, all seems to work, now moving to
> linux-next.

[    9s] /usr/bin/qemu-kvm -nodefaults -no-reboot -nographic -vga none -cpu host -object rng-random,filename=/dev/random,id=rng0 -device virtio-rng-pci,rng=rng0 -runas qemu -net none -kernel /var/cache/obs/worker/root_2/.mount/boot/kernel -initrd /var/cache/obs/worker/root_2/.mount/boot/initrd -append root=/dev/disk/by-id/virtio-0 rootfstype=ext4 rootflags=noatime ext4.allow_unsupported=1 mitigations=off panic=1 quiet no-kvmclock elevator=noop nmi_watchdog=0 rw rd.driver.pre=binfmt_misc console=ttyS0 init=/.build/build -m 8192 -drive file=/var/cache/obs/worker/root_2/root,format=raw,if=none,id=disk,cache=unsafe -device virtio-blk-pci,drive=disk,serial=0 -drive file=/var/cache/obs/worker/root_2/swap,format=raw,if=none,id=swap,cache=unsafe -device virtio-blk-pci,drive=swap,serial=1 -serial stdio -chardev socket,id=monitor,server,nowait,path=/var/cache/obs/worker/root_2/root.qemu/monitor -mon chardev=monitor,mode=readline -smp 8
[    9s] c[?7l[2J[0mSeaBIOS (version rel-1.14.0-0-g155821a-rebuilt.opensuse.org)
[   15s] Booting from ROM..c[?7l[2J### VM INTERACTION END ###
[   15s] 2nd stage started in virtual machine
[   15s] machine type: x86_64
[   15s] Linux version: 5.13.0-rc3-next-2[    5.210478] sysrq: Changing Loglevel
[   15s] 0210526-2.g3bd43[    5.211566] sysrq: Loglevel set to 4
[   15s] 9f-vanilla #1 SMP Thu May 27 18:27:17 UTC 2021 (3bd439f)
[   15s] Increasing log level from now on...
[   15s] Enable sysrq operations
[   15s] Setting up swapspace version 1, size = 2 GiB (2097147904 bytes)
[   15s] no label, UUID=6e22796c-6df5-4859-a291-24acde2e92e5
[   15s] swapon: /dev/vdb: found signature [pagesize=4096, signature=swap]
[   15s] swapon: /dev/vdb: pagesize=4096, swapsize=2097152000, devsize=2097152000
[   15s] swapon /dev/vdb
[   15s] WARNING: udev not running, creating extra device nodes
[   15s] logging output to //.build.log...

next-20210527 does not build becuase of missing symbol but next-20210526 built with the patched pahole boots.

Tested-by: Michal Suchánek <msuchanek@suse.de>

Thanks

Michal
>  
> > Thanks,
> > 
> > - Arnaldo
> >  
> > > Thanks
> > > 
> > > Michal
> > > 
> > > > 
> > > > Tested-by: Michal Suchánek <msuchanek@suse.de>
> > > > 
> > > > Thanks,
> > > > 
> > > > - Arnaldo
> > > >  
> > > > > > +++ b/btf_encoder.c
> > > > > > @@ -550,6 +551,7 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
> > > > > >  
> > > > > >  		/* addr has to be recorded before we follow spec */
> > > > > >  		addr = var->ip.addr;
> > > > > > +		dwarf_name = variable__name(var, cu);
> > > > > >  
> > > > > >  		/* DWARF takes into account .data..percpu section offset
> > > > > >  		 * within its segment, which for vmlinux is 0, but for kernel
> > > > > > @@ -582,11 +584,9 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
> > > > > >  		 *  modules per-CPU data section has non-zero offset so all
> > > > > >  		 *  per-CPU symbols have non-zero values.
> > > > > >  		 */
> > > > > > -		if (var->ip.addr == 0) {
> > > > > > -			dwarf_name = variable__name(var, cu);
> > > > > > +		if (var->ip.addr == 0)
> > > > > >  			if (!dwarf_name || strcmp(dwarf_name, name))
> > > > > >  				continue;
> > > > > > -		}
> > > > > >  
> > > > > >  		if (var->spec)
> > > > > >  			var = var->spec;
> > > > > > @@ -600,6 +600,13 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
> > > > > 
> > > > > I just changed the above hunk to be:
> > > > > 
> > > > > @@ -583,7 +585,6 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
> > > > >                  *  per-CPU symbols have non-zero values.
> > > > >                  */
> > > > >                 if (var->ip.addr == 0) {
> > > > > -                       dwarf_name = variable__name(var, cu);
> > > > >                         if (!dwarf_name || strcmp(dwarf_name, name))
> > > > >                                 continue;
> > > > >                 }
> > > > > 
> > > > > 
> > > > > Which is shorter and keeps the {} around a multi line if block, ok?
> > > > > 
> > > > > Thanks, applied!
> > > > > 
> > > > > - Arnaldo
> > > > 
> > > > -- 
> > > > 
> > > > - Arnaldo
> > 
> > -- 
> > 
> > - Arnaldo
> 
> -- 
> 
> - Arnaldo