On Tue, Apr 28, 2020 at 3:54 AM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote: > > Hi. > > I have a question about potential bit size > mismatch between vmlinux and bpfilter_umh. > > > net/bpfilter/bpfilter_umh is compiled for the > default machine bit of the compiler. > This may not match to the kernel bit size. > > > This happens in the following scenario. > > GCC can be compiled as bi-arch. > If you use GCC that defaults to 64-bit, > you can give -m32 flag to produce the 32 bit code. > > When you build the kernel for 32-bit, -m32 is > properly passed for building the kernel space objects. > However, it is missing while building the userspace > objects for bpfilter_umh. > > > For example, my build host is x86_64 Ubuntu. > > If I build the kernel for i386 > with CONFIG_BPFILTER_UMH=y, > the embedded bpfilter_umh is 64bit ELF. > > You can reproduce it by the following command on the > mainline kernel. > > masahiro@oscar:~/ref/linux$ make ARCH=i386 defconfig > masahiro@oscar:~/ref/linux$ scripts/config -e BPFILTER > masahiro@oscar:~/ref/linux$ scripts/config -e BPFILTER_UMH > masahiro@oscar:~/ref/linux$ make $(nproc) ARCH=i386 > ... > masahiro@oscar:~/ref/linux$ file vmlinux > vmlinux: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), > statically linked, > BuildID[sha1]=7ac691c67b4fe9b0cd46b45a2dc2d728d7d87686, not stripped > masahiro@oscar:~/ref/linux$ file net/bpfilter/bpfilter_umh > net/bpfilter/bpfilter_umh: ELF 64-bit LSB executable, x86-64, version > 1 (GNU/Linux), statically linked, > BuildID[sha1]=baf1ffe26f4c030a99a945fc22924c8c559e60ac, for GNU/Linux > 3.2.0, not stripped > > > > > At least, the build was successful, > but does this work at runtime? > > If this is a bug, I can fix it cleanly. > > I think the bit size of the user mode helper > should match to the kernel bit size. Is this correct? yes. they should match. In theory we can have -m32 umh running on 64-bit kernel, but I wouldn't bother adding support for such thing until there is a use case. Running 64-bit umh on 32-bit kernel is no go.
