Re: [PATCH v2 bpf-next] libbpf: btf_dump_type_data_check_overflow needs to consider BTF_MEMBER_BITFIELD_SIZE

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On 4/27/23 6:36 PM, Martin KaFai Lau wrote:
From: Martin KaFai Lau <martin.lau@xxxxxxxxxx>

The btf_dump/struct_data selftest is failing with:
test_btf_dump_struct_data:FAIL:unexpected return value dumping fs_context unexpected unexpected return value dumping fs_context: actual -7 != expected 264

The reason is in btf_dump_type_data_check_overflow(). It does not use
BTF_MEMBER_BITFIELD_SIZE from the struct's member (btf_member). Instead,
it is using the enum size which is 4. It had been working till the recent
commit 4e04143c869c ("fs_context: drop the unused lsm_flags member")
removed an integer member which also removed the 4 bytes padding at the end
of the fs_context. Missing this 4 bytes padding exposed this bug.
In particular, when btf_dump_type_data_check_overflow() reaches
the member 'phase', -E2BIG is returned.

The fix is to pass bit_sz to btf_dump_type_data_check_overflow().
In btf_dump_type_data_check_overflow(), it does a different size
check when bit_sz is not zero.

The current fs_context:

[3600] ENUM 'fs_context_purpose' encoding=UNSIGNED size=4 vlen=3
	'FS_CONTEXT_FOR_MOUNT' val=0
	'FS_CONTEXT_FOR_SUBMOUNT' val=1
	'FS_CONTEXT_FOR_RECONFIGURE' val=2
[3601] ENUM 'fs_context_phase' encoding=UNSIGNED size=4 vlen=7
	'FS_CONTEXT_CREATE_PARAMS' val=0
	'FS_CONTEXT_CREATING' val=1
	'FS_CONTEXT_AWAITING_MOUNT' val=2
	'FS_CONTEXT_AWAITING_RECONF' val=3
	'FS_CONTEXT_RECONF_PARAMS' val=4
	'FS_CONTEXT_RECONFIGURING' val=5
	'FS_CONTEXT_FAILED' val=6
[3602] STRUCT 'fs_context' size=264 vlen=21
	'ops' type_id=3603 bits_offset=0
	'uapi_mutex' type_id=235 bits_offset=64
	'fs_type' type_id=872 bits_offset=1216
	'fs_private' type_id=21 bits_offset=1280
	'sget_key' type_id=21 bits_offset=1344
	'root' type_id=781 bits_offset=1408
	'user_ns' type_id=251 bits_offset=1472
	'net_ns' type_id=984 bits_offset=1536
	'cred' type_id=1785 bits_offset=1600
	'log' type_id=3621 bits_offset=1664
	'source' type_id=42 bits_offset=1792
	'security' type_id=21 bits_offset=1856
	's_fs_info' type_id=21 bits_offset=1920
	'sb_flags' type_id=20 bits_offset=1984
	'sb_flags_mask' type_id=20 bits_offset=2016
	's_iflags' type_id=20 bits_offset=2048
	'purpose' type_id=3600 bits_offset=2080 bitfield_size=8
	'phase' type_id=3601 bits_offset=2088 bitfield_size=8
	'need_free' type_id=67 bits_offset=2096 bitfield_size=1
	'global' type_id=67 bits_offset=2097 bitfield_size=1
	'oldapi' type_id=67 bits_offset=2098 bitfield_size=1

Signed-off-by: Martin KaFai Lau <martin.lau@xxxxxxxxxx>

Acked-by: Yonghong Song <yhs@xxxxxx>



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