CVE-2024-42161

In the Linux kernel, the following vulnerability has been resolved:

bpf: Avoid uninitialized value in BPFCOREREAD_BITFIELD

[Changes from V1: - Use a default branch in the switch statement to initialize `val'.]

GCC warns that `val' may be used uninitialized in the BPFCREREADBITFIELD macro, defined in bpfcore_read.h as:

[...]
unsigned long long val;                           \
[...]                                     \
switch (__CORE_RELO(s, field, BYTE_SIZE)) {               \
case 1: val = *(const unsigned char *)p; break;               \
case 2: val = *(const unsigned short *)p; break;              \
case 4: val = *(const unsigned int *)p; break;                \
case 8: val = *(const unsigned long long *)p; break;              \
    }                                         \
[...]
val;                                      \
}                                     \

This patch adds a default entry in the switch statement that sets `val' to zero in order to avoid the warning, and random values to be used in case _builtinpreservefieldinfo returns unexpected values for BPFFIELDBYTE_SIZE.

Tested in bpf-next master. No regressions.