CVE-2024-35917: s390/bpf: Fix bpf_plt pointer arithmetic

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

s390/bpf: Fix bpfplt pointer arithmetic

Kui-Feng Lee reported a crash on s390x triggered by the dummystops/dummyinitptrarg test [1]:

[<0000000000000002>] 0x2 [<00000000009d5cde>] bpfstructopstestrun+0x156/0x250 [<000000000033145a>] sysbpf+0xa1a/0xd00 [<00000000003319dc>] s390xsysbpf+0x44/0x50 [<0000000000c4382c>] dosyscall+0x244/0x300 [<0000000000c59a40>] systemcall+0x70/0x98

This is caused by GCC moving memcpy() after assignments in bpfjitplt(), resulting in NULL pointers being written instead of the return and the target addresses.

Looking at the GCC internals, the reordering is allowed because the alias analysis thinks that the memcpy() destination and the assignments' left-hand-sides are based on different objects: newplt and bpfpltret/bpfplttarget respectively, and therefore they cannot alias.

This is in turn due to a violation of the C standard:

When two pointers are subtracted, both shall point to elements of the same array object, or one past the last element of the array object ...

From the C's perspective, bpfpltret and bpfplt are distinct objects and cannot be subtracted. In the practical terms, doing so confuses the GCC's alias analysis.

The code was written this way in order to let the C side know a few offsets defined in the assembly. While nice, this is by no means necessary. Fix the noncompliance by hardcoding these offsets.

[1] https://lore.kernel.org/bpf/c9923c1d-971d-4022-8dc8-1364e929d34c@gmail.com/