Re: [PATCH 1/2] module: Overwrite st_size instead of st_info

[Jessica Yu]

+++ 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.whitchurch@xxxxxxxx>
> > > ---
> > > 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.

Thanks,

Jessica