On Fri, Jul 19, 2024, Mirsad Todorovac wrote: > Hi, all! > > On linux-stable 6.10 vanilla tree, another NULL pointer is passed, which was detected > by the fortify-string.h mechanism. > > arch/x86/kvm/x86.c > ================== > > 13667 kvm_prepare_emulation_failure_exit(vcpu); > > calls > > 8796 __kvm_prepare_emulation_failure_exit(vcpu, NULL, 0); > > which calls > > 8790 prepare_emulation_failure_exit(vcpu, data, ndata, NULL, 0); > > Note here that data == NULL and ndata = 0. > > again data == NULL and ndata == 0, which passes unchanged all until > > 8773 memcpy(&run->internal.data[info_start + ARRAY_SIZE(info)], data, ndata * sizeof(data[0])); My reading of the C99 is that KVM's behavior is fine. Where an argument declared as size_t n specifies the length of the array for a function, n can have the value zero on a call to that function. Unless explicitly stated otherwise in the description of a particular function in this subclause, pointer arguments on such a call shall still have valid values, as described in 7.1.4. On such a call, a function that locates a character finds no occurrence, a function that compares two character sequences returns zero, and a function that copies characters copies zero characters. If the function copies zero characters, then there can't be a store to the NULL pointer, and if there's no store, there's no NULL pointer explosion. I suppose arguably one could argue the builtin memcpy() could deliberately fail on an invalid pointer, but that'd be rather ridiculous.
