bpf: Avoid uninitialized value in BPF_CORE_READ_BITFIELD

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

bpf: Avoid uninitialized value in BPF_CORE_READ_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

BPF_CRE_READ_BITFIELD macro, defined in bpf_core_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 __builtin_preserve_field_info returns unexpected values

for BPF_FIELD_BYTE_SIZE.

Tested in bpf-next master.

No regressions.