tunaichao
2e44ad8dbd
Building with -Dbpf=enabled -Dbpf_compiler=gcc (GCC's BPF backend) fails on
the static assertions in bpf-restrict-fsaccess.c, introduced in 68fe7fa4d6:
error: static assertion failed:
"offsetof(struct restrict_fsaccess_bss, initramfs_s_dev) ==
offsetof(typeof_field(struct restrict_fsaccess_bpf, bss[0]), initramfs_s_dev)"
The hand-written struct restrict_fsaccess_bss lists the BPF .bss globals in
source declaration order and asserts that its layout matches the skeleton's
generated bss struct. bpftool gen skeleton emits that struct from the BTF
.bss DATASEC, whose member order reflects the physical order the compiler
placed the variables, not the source order. clang preserves declaration
order, so the asserts pass; gcc reorders .bss globals, so initramfs_s_dev no
longer sits at offset 0 and the asserts fail.
This is more than a build break: restrict_fsaccess_clear_initramfs_trust()
clears initramfs_s_dev by mmap()ing the .bss map and storing 0 at a hardcoded
offset 0. Under the gcc layout that store would clobber the wrong global,
silently leaving the initramfs trust window open after switch_root instead of
closing it. The asserts were correctly catching this.
Fix it by deriving the offset from the generated skeleton instead of a mirror
struct: drop struct restrict_fsaccess_bss and the four field-order
assert_cc()s, take INITRAMFS_S_DEV_OFF from the skeleton's bss struct
(offsetof(typeof_field(struct restrict_fsaccess_bpf, bss[0]),
initramfs_s_dev)), and store at p + INITRAMFS_S_DEV_OFF. The offset is a
compile-time constant, so clang (offset 0) is unchanged while gcc tracks the
real layout. A retained assert_cc() documents the 4-byte alignment the
single-store atomicity relies on.
Fixes: #42689
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