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Dereferencing pointes via [0] fails to load the program #3016

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viktormalik opened this issue Feb 22, 2024 · 0 comments · Fixed by #3024
Closed

Dereferencing pointes via [0] fails to load the program #3016

viktormalik opened this issue Feb 22, 2024 · 0 comments · Fixed by #3024
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@viktormalik
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When trying to dereference a kernel pointer with [0], the BPF verifier refuses to load the program.

This program:

#! /usr/bin/bpftrace

struct ib_gid_table_entry {
        struct kref                     kref;
        struct work_struct              del_work;
        struct ib_gid_attr              attr;
};

kfunc:find_gid
{
        print(args.table->data_vec[0]->attr.gid.raw);
}

triggers a verifier error:

$ sudo test.bt
INFO: node count: 15
Attaching 1 probe...

Error log: 
reg type unsupported for arg#0 function kfunc_ib_core_find_gid#22
0: R1=ctx(off=0,imm=0) R10=fp0
0: (79) r1 = *(u64 *)(r1 +0)
func 'find_gid' arg0 has btf_id 134135 type STRUCT 'ib_gid_table'
1: R1_w=ptr_ib_gid_table(off=0,imm=0)
1: (79) r1 = *(u64 *)(r1 +48)         ; R1_w=scalar()
2: (79) r3 = *(u64 *)(r1 +0)
R1 invalid mem access 'scalar'
processed 3 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0

ERROR: Error loading program: kfunc:ib_core:find_gid

However, this works fine:

#! /usr/bin/bpftrace

struct ib_gid_table_entry {
        struct kref                     kref;
        struct work_struct              del_work;
        struct ib_gid_attr              attr;
};

kfunc:find_gid
{
        print((*args.table->data_vec)->attr.gid.raw);
}

Interestingly, the second example works even without the definition of struct ib_gid_table_entry (it's pulled from BTF of the ib_core module), omitting the struct def from the first example yields a different error:

$ bpftrace -e 'kfunc:find_gid { print(args.table->data_vec[0]->attr.gid.raw); }'
stdin:1:18-53: ERROR: Struct/union of type 'struct ib_gid_table_entry' does not contain a field named 'attr'
kfunc:find_gid { print(args.table->data_vec[0]->attr.gid.raw); }
                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The issues are very likely connected (I suspect incorrect handling of [0] for pointer types in both field analyser and codegen).
@viktormalik viktormalik added the bug Something isn't working label Feb 22, 2024
@viktormalik viktormalik self-assigned this Feb 22, 2024
@viktormalik viktormalik mentioned this issue Feb 29, 2024
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Fix error in dereferencing kernel double pointers viktormalik/bpftrace
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