On Mon, Sep 27, 2021 at 09:37:59AM -0700, Luis Chamberlain wrote: > There is quite a bit of tribal knowledge around proper use of > try_module_get() and that it must be used only in a context which > can ensure the module won't be gone during the operation. Document > this little bit of tribal knowledge. > > I'm extending this tribal knowledge with new developments which it > seems some folks do not yet believe to be true: we can be sure a > module will exist during the lifetime of a sysfs file operation. > For proof, refer to test_sysfs test #32: > > ./tools/testing/selftests/sysfs/sysfs.sh -t 0032 > > Without this being true, the write would fail or worse, > a crash would happen, in this test. It does not. > > Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx> > --- > include/linux/module.h | 34 ++++++++++++++++++++++++++++++++-- > 1 file changed, 32 insertions(+), 2 deletions(-) > > diff --git a/include/linux/module.h b/include/linux/module.h > index c9f1200b2312..22eacd5e1e85 100644 > --- a/include/linux/module.h > +++ b/include/linux/module.h > @@ -609,10 +609,40 @@ void symbol_put_addr(void *addr); > to handle the error case (which only happens with rmmod --wait). */ > extern void __module_get(struct module *module); > > -/* This is the Right Way to get a module: if it fails, it's being removed, > - * so pretend it's not there. */ > +/** > + * try_module_get() - yields to module removal and bumps refcnt otherwise I find this hard to parse. How about: "Take module refcount unless module is being removed" > + * @module: the module we should check for > + * > + * This can be used to try to bump the reference count of a module, so to > + * prevent module removal. The reference count of a module is not allowed > + * to be incremented if the module is already being removed. This I understand. > + * > + * Care must be taken to ensure the module cannot be removed during the call to > + * try_module_get(). This can be done by having another entity other than the > + * module itself increment the module reference count, or through some other > + * means which guarantees the module could not be removed during an operation. > + * An example of this later case is using try_module_get() in a sysfs file > + * which the module created. The sysfs store / read file operations are > + * gauranteed to exist through the use of kernfs's active reference (see > + * kernfs_active()). If a sysfs file operation is being run, the module which > + * created it must still exist as the module is in charge of removing the same > + * sysfs file being read. Also, a sysfs / kernfs file removal cannot happen > + * unless the same file is not active. I can't understand this paragraph at all. "Care must be taken ..."? Why? Shouldn't callers of try_module_get() be satisfied with the results? I don't follow the example at all. It seems to just say "sysfs store/read functions don't need try_module_get() because whatever opened the sysfs file is already keeping the module referenced." ? > + * > + * One of the real values to try_module_get() is the module_is_live() check > + * which ensures this the caller of try_module_get() can yield to userspace > + * module removal requests and fail whatever it was about to process. Please document the return value explicitly. > + */ > extern bool try_module_get(struct module *module); > > +/** > + * module_put() - release a reference count to a module > + * @module: the module we should release a reference count for > + * > + * If you successfully bump a reference count to a module with try_module_get(), > + * when you are finished you must call module_put() to release that reference > + * count. > + */ > extern void module_put(struct module *module); > > #else /*!CONFIG_MODULE_UNLOAD*/ > -- > 2.30.2 > -- Kees Cook
