On 5/8/24 3:35 AM, Jose E. Marchesi wrote:
[Changes from V2:
- no-strict-aliasing is only applied when building with GCC.
- cpumask_failure.c is excluded, as it doesn't use __imm_insn.]
The __imm_insn macro is defined in bpf_misc.h as:
#define __imm_insn(name, expr) [name]"i"(*(long *)&(expr))
This may lead to type-punning and strict aliasing rules violations in
it's typical usage where the address of a struct bpf_insn is passed as
expr, like in:
__imm_insn(st_mem,
BPF_ST_MEM(BPF_W, BPF_REG_1, offsetof(struct __sk_buff, mark), 42))
Where:
#define BPF_ST_MEM(SIZE, DST, OFF, IMM) \
((struct bpf_insn) { \
.code = BPF_ST | BPF_SIZE(SIZE) | BPF_MEM, \
.dst_reg = DST, \
.src_reg = 0, \
.off = OFF, \
.imm = IMM })
In all the actual instances of this in the BPF selftests the value is
fed to a volatile asm statement as soon as it gets read from memory,
and thus it is unlikely anti-aliasing rules breakage may lead to
misguided optimizations.
However, GCC detects the potential problem (indirectly) by issuing a
warning stating that a temporary <Uxxxxxx> is used uninitialized,
where the temporary corresponds to the memory read by *(long *).
This patch adds -fno-strict-aliasing to the compilation flags of the
particular selftests that do type punning via __imm_insn, only for
GCC.
Tested in master bpf-next.
No regressions.
Signed-off-by: Jose E. Marchesi <jose.marchesi@xxxxxxxxxx>
Cc: david.faust@xxxxxxxxxx
Cc: cupertino.miranda@xxxxxxxxxx
Cc: Yonghong Song <yonghong.song@xxxxxxxxx>
Cc: Eduard Zingerman <eddyz87@xxxxxxxxx>
Acked-by: Yonghong Song <yonghong.song@xxxxxxxxx>
