On Mon, Oct 07, 2019 at 04:46:59PM +0100, Will Deacon wrote: > Having the refcount saturation and warnings inline bloats the text, > despite the fact that these paths should never be executed in normal > operation. > > Move the refcount saturation and warnings out of line to reduce the > image size when refcount_t checking is enabled. Relative to an x86_64 > defconfig, the sizes reported by bloat-o-meter are: > > # defconfig+REFCOUNT_FULL, inline saturation (i.e. before this patch) > Total: Before=14762076, After=14915442, chg +1.04% > > # defconfig+REFCOUNT_FULL, out-of-line saturation (i.e. after this patch) > Total: Before=14762076, After=14835497, chg +0.50%
The downside of this change is that this means we get one warning per refcount_saturation_type, where as before it was once per refcount usage. I think, ultimately, this is okay, but it is a behavioral change that might be worth pointing out. > > Cc: Ingo Molnar <[email protected]> > Cc: Elena Reshetova <[email protected]> > Cc: Peter Zijlstra <[email protected]> > Cc: Ard Biesheuvel <[email protected]> > Cc: Kees Cook <[email protected]> > Signed-off-by: Will Deacon <[email protected]> > --- > include/linux/refcount.h | 39 ++++++++++++++++++++------------------- > lib/refcount.c | 28 ++++++++++++++++++++++++++++ > 2 files changed, 48 insertions(+), 19 deletions(-) > > diff --git a/include/linux/refcount.h b/include/linux/refcount.h > index 7f9aa6511142..40ddff74a96c 100644 > --- a/include/linux/refcount.h > +++ b/include/linux/refcount.h > @@ -23,6 +23,16 @@ typedef struct refcount_struct { > > #define REFCOUNT_INIT(n) { .refs = ATOMIC_INIT(n), } > > +enum refcount_saturation_type { > + REFCOUNT_ADD_NOT_ZERO_OVF, > + REFCOUNT_ADD_OVF, > + REFCOUNT_ADD_UAF, > + REFCOUNT_SUB_UAF, > + REFCOUNT_DEC_LEAK, > +}; > + > +void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t); > + > /** > * refcount_set - set a refcount's value > * @r: the refcount > @@ -116,10 +126,8 @@ static inline __must_check bool > refcount_add_not_zero(int i, refcount_t *r) > break; > } while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i)); > > - if (unlikely(old < 0 || old + i < 0)) { > - refcount_set(r, REFCOUNT_SATURATED); > - WARN_ONCE(1, "refcount_t: saturated; leaking memory.\n"); > - } > + if (unlikely(old < 0 || old + i < 0)) > + refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF); > > return old; > } > @@ -144,11 +152,10 @@ static inline void refcount_add(int i, refcount_t *r) > { > int old = atomic_fetch_add_relaxed(i, &r->refs); > > - WARN_ONCE(!old, "refcount_t: addition on 0; use-after-free.\n"); > - if (unlikely(old <= 0 || old + i <= 0)) { > - refcount_set(r, REFCOUNT_SATURATED); > - WARN_ONCE(old, "refcount_t: saturated; leaking memory.\n"); > - } > + if (unlikely(!old)) > + refcount_warn_saturate(r, REFCOUNT_ADD_UAF); > + else if (unlikely(old < 0 || old + i < 0)) > + refcount_warn_saturate(r, REFCOUNT_ADD_OVF); > } > > /** > @@ -215,10 +222,8 @@ static inline __must_check bool > refcount_sub_and_test(int i, refcount_t *r) > return true; > } > > - if (unlikely(old - i < 0)) { > - refcount_set(r, REFCOUNT_SATURATED); > - WARN_ONCE(1, "refcount_t: underflow; use-after-free.\n"); > - } > + if (unlikely(old - i < 0)) > + refcount_warn_saturate(r, REFCOUNT_SUB_UAF); > > return false; > } > @@ -253,12 +258,8 @@ static inline __must_check bool > refcount_dec_and_test(refcount_t *r) > */ > static inline void refcount_dec(refcount_t *r) > { > - int old = atomic_fetch_sub_release(1, &r->refs); > - > - if (unlikely(old <= 1)) { > - refcount_set(r, REFCOUNT_SATURATED); > - WARN_ONCE(1, "refcount_t: decrement hit 0; leaking memory.\n"); > - } > + if (unlikely(atomic_fetch_sub_release(1, &r->refs) <= 1)) > + refcount_warn_saturate(r, REFCOUNT_DEC_LEAK); > } > #else /* CONFIG_REFCOUNT_FULL */ > > diff --git a/lib/refcount.c b/lib/refcount.c > index 3a534fbebdcc..6a61d39f9390 100644 > --- a/lib/refcount.c > +++ b/lib/refcount.c > @@ -8,6 +8,34 @@ > #include <linux/spinlock.h> > #include <linux/bug.h> > > +#define REFCOUNT_WARN(str) WARN_ONCE(1, "refcount_t: " str ".\n") This define seems like overkill for just adding a prefix to 5 uses... > + > +void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t) > +{ > + refcount_set(r, REFCOUNT_SATURATED); > + > + switch (t) { > + case REFCOUNT_ADD_NOT_ZERO_OVF: > + REFCOUNT_WARN("saturated; leaking memory"); > + break; > + case REFCOUNT_ADD_OVF: > + REFCOUNT_WARN("saturated; leaking memory"); > + break; > + case REFCOUNT_ADD_UAF: > + REFCOUNT_WARN("addition on 0; use-after-free"); > + break; > + case REFCOUNT_SUB_UAF: > + REFCOUNT_WARN("underflow; use-after-free"); > + break; > + case REFCOUNT_DEC_LEAK: > + REFCOUNT_WARN("decrement hit 0; leaking memory"); Even the longest doesn't line wrap: WARN_ONCE(1, "refcount_t: decrement hit 0; leaking memory\n"); > + break; > + default: > + WARN_ON(1); This deserves a string too, yes? WARN_ONCE(1, "refcount_t: unknown saturation event!?\n"); > + } > +} > +EXPORT_SYMBOL(refcount_warn_saturate); > + > /** > * refcount_dec_if_one - decrement a refcount if it is 1 > * @r: the refcount > -- > 2.23.0.581.g78d2f28ef7-goog > Otherwise, okay, I grudgingly accept the loss of warnings when running the lkdtm tests in order to gain back some text size... :) -- Kees Cook
