On Thu, Nov 22, 2018 at 06:40:45PM +0100, Ard Biesheuvel wrote: > On Thu, 22 Nov 2018 at 17:29, Jessica Yu <j...@kernel.org> wrote: > > > > +++ Vincent Whitchurch [22/11/18 13:24 +0100]: > > >On Thu, Nov 22, 2018 at 12:01:54PM +0000, Dave Martin wrote: > > >> On Mon, Nov 19, 2018 at 05:25:12PM +0100, Vincent Whitchurch wrote: > > >> > st_info is currently overwritten after relocation and used to store the > > >> > elf_type(). However, we're going to need it fix kallsyms on ARM's > > >> > Thumb-2 kernels, so preserve st_info and overwrite the st_size field > > >> > instead. st_size is neither used by the module core nor by any > > >> > architecture. > > >> > > > >> > Signed-off-by: Vincent Whitchurch <vincent.whitchu...@axis.com> > > >> > --- > > >> > v4: Split out to separate patch. Use st_size instead of st_other. > > >> > v1-v3: See PATCH 2/2 > > >> > > > >> > kernel/module.c | 4 ++-- > > >> > 1 file changed, 2 insertions(+), 2 deletions(-) > > >> > > > >> > diff --git a/kernel/module.c b/kernel/module.c > > >> > index 49a405891587..3d86a38b580c 100644 > > >> > --- a/kernel/module.c > > >> > +++ b/kernel/module.c > > >> > @@ -2682,7 +2682,7 @@ static void add_kallsyms(struct module *mod, > > >> > const struct load_info *info) > > >> > > > >> > /* Set types up while we still have access to sections. */ > > >> > for (i = 0; i < mod->kallsyms->num_symtab; i++) > > >> > - mod->kallsyms->symtab[i].st_info > > >> > + mod->kallsyms->symtab[i].st_size > > >> > = elf_type(&mod->kallsyms->symtab[i], info); > > >> > > > >> > /* Now populate the cut down core kallsyms for after init. */ > > >> > @@ -4061,7 +4061,7 @@ int module_get_kallsym(unsigned int symnum, > > >> > unsigned long *value, char *type, > > >> > kallsyms = rcu_dereference_sched(mod->kallsyms); > > >> > if (symnum < kallsyms->num_symtab) { > > >> > *value = kallsyms->symtab[symnum].st_value; > > >> > - *type = kallsyms->symtab[symnum].st_info; > > >> > + *type = kallsyms->symtab[symnum].st_size; > > >> > strlcpy(name, symname(kallsyms, symnum), > > >> > KSYM_NAME_LEN); > > >> > strlcpy(module_name, mod->name, MODULE_NAME_LEN); > > >> > *exported = is_exported(name, *value, mod); > > >> > > >> This is fine if st_size is really unused, but how sure are you of that? > > >> > > >> grepping for st_size throws up some hits that appear ELF-related, but > > >> I've not investigated them in detail. > > >> > > >> (The fact that struct stat has an identically named field throws up > > >> a load of false positives too.) > > > > > >$ git describe --tags > > >v4.20-rc3-93-g92b419289cee > > > > > >$ rg -m1 '[\.>]st_size' --iglob '!**/tools/**' --iglob '!**/vdso*' > > >--iglob '!**/scripts/**' --iglob '!**/usr/**' --iglob '!**/samples/**' | > > >cat > > >| kernel/kexec_file.c: if (sym->st_size != size) { > > > > > >Symbols in kexec kernel. > > > > > >| fs/stat.c: tmp.st_size = stat->size; > > >| Documentation/networking/tls.txt: sendfile(sock, file, &offset, > > >stat.st_size); > > >| net/9p/client.c: ret->st_rdev, ret->st_size, ret->st_blksize, > > >| net/9p/protocol.c: &stbuf->st_rdev, > > >&stbuf->st_size, > > >| fs/9p/vfs_inode_dotl.c: i_size_write(inode, stat->st_size); > > >| fs/hostfs/hostfs_user.c: p->size = buf->st_size; > > >| arch/powerpc/boot/mktree.c: nblks = (st.st_size + IMGBLK) / IMGBLK; > > >| arch/alpha/kernel/osf_sys.c: tmp.st_size = lstat->size; > > >| arch/x86/ia32/sys_ia32.c: __put_user(stat->size, &ubuf->st_size) > > >|| > > > > > >Not Elf_Sym. > > > > > >| arch/x86/kernel/machine_kexec_64.c: sym->st_size); > > > > > >Symbols in kexec kernel. > > > > > >| arch/sparc/boot/piggyback.c: st4(buffer + 12, s.st_size); > > >| arch/sparc/kernel/sys_sparc32.c: err |= put_user(stat->size, > > >&statbuf->st_size); > > >| arch/um/os-Linux/file.c: .ust_size = src->st_size, /* > > >total size, in bytes */ > > >| arch/um/os-Linux/start_up.c: size = (buf.st_size + UM_KERN_PAGE_SIZE) & > > >~(UM_KERN_PAGE_SIZE - 1); > > >| arch/s390/kernel/compat_linux.c: tmp.st_size = stat->size; > > >| arch/arm/kernel/sys_oabi-compat.c: tmp.st_size = stat->size; > > >| arch/mips/boot/compressed/calc_vmlinuz_load_addr.c: vmlinux_size = > > >(uint64_t)sb.st_size; > > >| drivers/net/ethernet/marvell/sky2.c: hw->st_idx = > > >RING_NEXT(hw->st_idx, hw->st_size); > > > > > >Not Elf_Sym. > > > > [ added Miroslav to CC, just in case he would like to check :) ] > > > > I have just double checked as well, and am fairly certain that the > > Elf_Sym st_size field is not used to apply module relocations in any > > arches, and it is not used in the core module loader nor in the module > > kallsyms code. We'd like to avoid overwriting st_info in any case, to > > fix kallsyms on Thumb-2 and also so that livepatch won't run into any > > issues with delayed relocations, should livepatch support ever expand > > to arches (e.g., arm) that rely on st_info for module relocations. > > > > Also note that st_size cannot be relied upon in general, since we > overwrite the addresses of undefined symbols in a module's symbol > table when resolve them against ksyms, while the st_size field is kept > at 0. At relocation time, we don't really distinguish anymore between > local and external module symbols, and so relying on st_size to be > accurate would definitely break things.
Umm. Undefined symbols of course have a zero size, because it's not known how large the definition that the symbol refers to is. However, the non-undefined symbols should have a size with them - anything emitted by the compiler should, but only if .size has been used in the assembler will assembly have correct sizes. Aren't SHN_UNDEF symbols ignored, except when applying the relocations, where we rely on st_value to be set on SHN_UNDEF symbols? -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up