Messages in this thread | | | Date | Mon, 27 Feb 2012 12:15:38 -0800 | Subject | Re: [PATCH v11 06/12] seccomp: add system call filtering using BPF | From | Kees Cook <> |
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On Mon, Feb 27, 2012 at 11:54 AM, Will Drewry <wad@chromium.org> wrote: > On Mon, Feb 27, 2012 at 11:09 AM, Oleg Nesterov <oleg@redhat.com> wrote: >> On 02/24, Will Drewry wrote: >>> +static struct seccomp_filter *get_seccomp_filter(struct seccomp_filter *orig) >>> +{ >>> + if (!orig) >>> + return NULL; >>> + /* Reference count is bounded by the number of total processes. */ >>> + atomic_inc(&orig->usage); >>> + return orig; >>> +} >>> ... >>> +void copy_seccomp(struct seccomp *child, const struct seccomp *parent) >>> +{ >>> + /* Other fields are handled by dup_task_struct. */ >>> + child->filter = get_seccomp_filter(parent->filter); >>> +} >> >> This is purely cosmetic, but imho looks a bit confusing. >> >> We do not copy seccomp->mode and this is correct, it was already copied >> implicitely. So why do we copy ->filter? This is not "symmetrical", afaics >> you can simply do >> >> void copy_seccomp(struct seccomp *child) >> { >> if (child->filter) >> atomic_inc(child->filter->usage); >> >> But once again, this is cosmetic, feel free to ignore. > > Right now get_seccomp_filter does the NULL check, so really this could > be reduced to adding an external get_seccomp_filter(p->seccomp.filter) > in place of copy_seccomp(). > > As to removing the extra arg, that should be fine since the parent > can't drop its refcount when copy_seccomp is called. At the very > least, I can make that change so it reads more cleanly.
I had various conflicting thoughts while looking over the refcounting:
- get_seccomp_filter is defined static, and has a single caller: copy_seccomp() - put isn't static, and has a single caller: kernel/fork.c:free_task() - having only get_/put_ touch ->usage seems cleaner to me - seccomp_attach_filter touches ->usage without get_seccomp_filter - having the initializing routine use atomic_set(..., 1) is a common pattern
In a fit of extreme bike-shedding, I can't decide which is more sensible:
- rename put_seccomp_filter to free_seccomp_filter and inline the get_seccomp_filter logic into copy_seccomp().
or
- create a wrapper for put_seccomp_filter named free_seccomp_filter so that get_/put_ can both be static.
-Kees
-- Kees Cook ChromeOS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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