Re: [PATCH] fgraph: record function return value

[Matthew Wilcox]

On Sat, Jan 12, 2019 at 02:57:01PM +0800, Changbin Du wrote:
> This patch adds a new trace option 'funcgraph-retval' and is disabled by
> default. When this option is enabled, fgraph tracer will show the return
> value of each function. This is useful to find/analyze a original error
> source in a call graph.
> 
> One limitation is that kernel doesn't know the prototype of functions. So
> fgraph assumes all functions have a retvalue of type int. You must ignore
> the value of *void* function. And if the retvalue looks like an error code
> then both hexadecimal and decimal number are displayed.

I don't think we can do this.  You're leaking a _lot_ of kernel addresses
this way, and we've been trying very hard to avoid doing that because
it gives a lot of information to attackers.

Something more clever that prints out only errors (ie IS_ERR_VALUE())
might be acceptable.  I would think printing return values that are
between 0 and 4095 should also be OK since they can't be real pointers.
We'd leak whether a function called kmalloc(0, x) since that returns
16, but that seems like a not-very-useful information leak.

>  3)   0.247 us    |    mutex_unlock(); /* ret=0xffff8880738ed040 */
>  3)               |    kvm_arch_vcpu_create() {
>  3)               |      vmx_create_vcpu() {
>  3) + 17.969 us   |        kmem_cache_alloc(); /* ret=0xffff88813a980040 */
>  3) + 15.948 us   |        kmem_cache_alloc(); /* ret=0xffff88813aa99200 */
>  3)   0.653 us    |        allocate_vpid.part.88(); /* ret=0x1 */
>  3)   6.964 us    |        kvm_vcpu_init(); /* ret=0xfffffffb */
>  3)   0.323 us    |        free_vpid.part.89(); /* ret=0x1 */
>  3)   9.985 us    |        kmem_cache_free(); /* ret=0x80000000 */
>  3)   9.491 us    |        kmem_cache_free(); /* ret=0x80000000 */
>  3) + 69.858 us   |      } /* ret=0xfffffffffffffffb/-5 */
>  3) + 70.631 us   |    } /* ret=0xfffffffffffffffb/-5 */
>  3)               |    mutex_lock() {
>  3)               |      _cond_resched() {
>  3)   0.199 us    |        rcu_all_qs(); /* ret=0x80000000 */
>  3)   0.594 us    |      } /* ret=0x0 */
>  3)   1.067 us    |    } /* ret=0x0 */
>  3)   0.337 us    |    mutex_unlock(); /* ret=0xffff8880738ed040 */
>  3) + 92.730 us   |  } /* ret=0xfffffffffffffffb/-5 */