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

[Martin KaFai Lau <martin.lau@linux.dev>]

On 4/27/23 6:36 PM, Martin KaFai Lau wrote:
> From: Martin KaFai Lau <martin.lau@kernel.org>
> 
> 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@kernel.org>
Missed the fixes tag.

Fixes: 920d16af9b42 ("libbpf: BTF dumper support for typed data")