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bpf: avoid UB in usages of the __imm_insn macro #6985

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bpf: avoid UB in usages of the __imm_insn macro #6985

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Pull request for series with
subject: bpf: avoid UB in usages of the __imm_insn macro
version: 1
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=851140

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Upstream branch: 329a672
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=851140
version: 1

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Upstream branch: 329a672
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=851201
version: 2

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Upstream branch: 75b0fbf
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=851201
version: 2

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Upstream branch: 93d1c2d
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=851201
version: 2

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Upstream branch: 7e2c7a3
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=851201
version: 2

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Upstream branch: 7e2c7a3
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=851521
version: 3

@jemarch
bpf: avoid UB in usages of the __imm_insn macro
e43e59c 
[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@oracle.com>
Cc: david.faust@oracle.com
Cc: cupertino.miranda@oracle.com
Cc: Yonghong Song <yonghong.song@linux.dev>
Cc: Eduard Zingerman <eddyz87@gmail.com>
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Upstream branch: e612b5c
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=851521
version: 3

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At least one diff in series https://patchwork.kernel.org/project/netdevbpf/list/?series=851521 irrelevant now. Closing PR.

@kernel-patches-daemon-bpf kernel-patches-daemon-bpf bot deleted the series/851140=>bpf-next branch May 8, 2024 17:02
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