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SubjectRe: [PATCH] fgraph: record function return value
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 */

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