On 10/7/20 7:01 AM, Luigi Rizzo wrote:
I am experiencing some weirdness in global variables handling
in bpftool and libbpf, as described below.
This happens happen with code in foo_bpf.c compiled with
clang-10 -O2 -Wall -Werror -target bpf ...
and subsequently exported with
bpftool gen skeleton ...
(i have tried bpftool 5.8.7 and 5.9.0-rc6)
1. uninitialized globals are not recognised
The following code in the bpf program
int x;
SEC("fentry/bar")
int BPF_PROG(bar) { return 0;}
compiles ok but bpftool then complains
libbpf: prog 'bar': invalid relo against 'x' in special section
0xfff2; forgot to initialize global var?..
The error disappears if I initialize x=0 or x=1
(in the skeleton, x=0 ends up in .bss, x=1 ends up in .data)
Yes, this is a particular issue with llvm10 which without "=0", put
into "common" section which we do not support. The issue is correct
in llvm11 if I remember correctly. But please just use "x = 0" it works
for all versions of compilers.
2. .bss overrides from userspace are not seen in bpf at runtime
In foo_bpf.c I have "int x = 0;"
In the userspace program, before foo_bpf__load(), I do
obj->bss->x = 1
but after attach, the bpf code does not see the change, ie
"if (x == 0) { .. } else { .. }"
always takes the first branch.
If I initialize "int x = 2" and then do
obj->data->x = 1
the update is seen correctly ie
"if (x == 2) { .. } else { .. }"
takes one or the other depending on whether userspace overrides
the value before foo_bpf__load()
3. .data overrides do not seem to work for non-scalar types
In foo_bpf.c I have
struct one { int a; }; // type also visible to userspace
struct one x { .a = 2 }; // avoid bugs #1 and #2
If in userspace I do
obj->data->x.a = 1
the update is not seen in the kernel, ie
"if (x.a == 2) { .. } else { .. }"
always takes the first branch
Are these known issues ?
thanks
luigi
