Re: [PATCH] KVM: selftests: Add 'malloc' failure check in test_vmx_nested_state

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 23, 2024, Andrew Jones wrote:
> On Tue, Apr 23, 2024 at 07:56:01AM -0700, Sean Christopherson wrote:
> > +others
> > 
> > On Tue, Apr 23, 2024, Markus Elfring wrote:
> > > …
> > > > This patch will add the malloc failure checking
> > > …
> > > 
> > > * Please use a corresponding imperative wording for the change description.
> > > 
> > > * Would you like to add the tag “Fixes” accordingly?
> > 
> > Nah, don't bother with Fixes.  OOM will cause the test to fail regardless, the
> > fact that it gets an assert instead a NULL pointer deref is nice to have, but by
> > no means does it fix a bug.
> > 
> > > > +++ b/tools/testing/selftests/kvm/x86_64/vmx_set_nested_state_test.c
> > > > @@ -91,6 +91,7 @@ void test_vmx_nested_state(struct kvm_vcpu *vcpu)
> > > >  	const int state_sz = sizeof(struct kvm_nested_state) + getpagesize();
> > > >  	struct kvm_nested_state *state =
> > > >  		(struct kvm_nested_state *)malloc(state_sz);
> > > > +	TEST_ASSERT(state, "-ENOMEM when allocating kvm state");
> > > …
> > > 
> > > Can “errno” be relevant for the error message construction?
> > 
> > Probably not, but there's also no reason to assume ENOMEM.  TEST_ASSERT() spits
> > out the actual errno, and we can just say something like "malloc() failed for
> > blah blah blah".  
> > 
> > But rather than keeping playing whack-a-mole, what if we add macros to perform
> > allocations and assert on the result?  I have zero interest in chasing down all
> > of the "unsafe" allocations, and odds are very good that we'll collectively fail
> > to enforce checking on new code.
> > 
> > E.g. something like (obviously won't compile, just for demonstration purposes)
> > 
> > #define kvm_malloc(x)
> > ({
> > 	void *__ret;
> > 
> > 	__ret  = malloc(x);
> > 	TEST_ASSERT(__ret, "Failed malloc(" #x ")\n");
> > 	__ret;
> > })
> > 
> > #define kvm_calloc(x, y)
> > ({
> > 	void *__ret;
> > 
> > 	__ret  = calloc(x, y);
> > 	TEST_ASSERT(__ret, "Failed calloc(" #x ", " #y ")\n");
> > 	__ret;
> > })
> 
> Sounds good to me, but I'd call them test_malloc, test_calloc, etc. and
> put them in include/test_util.h

Possibly terrible idea: what if we used kmalloc() and kcalloc()?  K is for KVM :-)

I like test_* more than kvm_*, but I'm mildly concerned that readers will be
confused by "test", e.g. initially thinking the "test" means it's just "testing"
if allocation is possible.

The obvious counter-argument is that people might also get tripped by kmalloc(),
e.g. thinking that selftests is somehow doing a kernel allocation.

I almost wonder if we should just pick a prefix that's less obviously connected
to KVM and/or selftests, but unique and short.

Hmm, tmalloc(), i.e t[est]malloc()?  tcalloc() gets a bit close to Google's
TCMalloc[*], but I suspect that any confusion would be entirely limited to
Googlers, and I'll volunteer us to suck it up and deal with it :-)

[*] https://github.com/google/tcmalloc