On 2/4/22 3:11 AM, Timo Beckers wrote:
On 2/3/22 03:11, Yonghong Song wrote:
On 2/2/22 5:47 AM, Timo Beckers wrote:
On 2/2/22 08:17, Yonghong Song wrote:
On 2/1/22 10:07 AM, Vincent Li wrote:
On Fri, Jan 28, 2022 at 10:27 AM Vincent Li <vincent.mc.li@xxxxxxxxx> wrote:
On Thu, Jan 27, 2022 at 5:50 PM Yonghong Song <yhs@xxxxxx> wrote:
On 1/25/22 12:32 PM, Vincent Li wrote:
On Tue, Jan 25, 2022 at 9:52 AM Vincent Li <vincent.mc.li@xxxxxxxxx> wrote:
this is macro I suspected in my implementation that could cause issue with BTF
#define ENABLE_VTEP 1
#define VTEP_ENDPOINT (__u32[]){0xec48a90a, 0xee48a90a, 0x1f48a90a,
0x2048a90a, }
#define VTEP_MAC (__u64[]){0x562e984c3682, 0x582e984c3682,
0x5eaaed93fdf2, 0x5faaed93fdf2, }
#define VTEP_NUMS 4
On Tue, Jan 25, 2022 at 9:38 AM Vincent Li <vincent.mc.li@xxxxxxxxx> wrote:
Hi
While developing Cilium VTEP integration feature
https://github.com/cilium/cilium/pull/17370, I found a strange issue
that seems related to BTF and probably caused by my specific
implementation, the issue is described in
https://github.com/cilium/cilium/issues/18616, I don't know much about
BTF and not sure if my implementation is seriously flawed or just some
implementation bug or maybe not compatible with BTF. Strangely, the
issue appears related to number of VTEPs I use, no problem with 1 or 2
VTEP, 3, 4 VTEPs will have problem with BTF, any guidance from BTF
experts are appreciated :-).
Thanks
Vincent
Sorry for previous top post
it looks the compiler compiles the cilium bpf_lxc.c to bpf_lxc.o
differently and added " [21] .rodata.cst32 PROGBITS
0000000000000000 00011e68" when following macro exceeded 2 members
#define VTEP_ENDPOINT (__u32[]){0xec48a90a, 0xee48a90a, 0x1f48a90a,
0x2048a90a, }
no ".rodata.cst32" compiled in bpf_lxc.o when above VTEP_ENDPOINT
member <=2. any reason why compiler would do that?
Regarding to why compiler generates .rodata.cst32, the reason is
you have some 32-byte constants which needs to be saved somewhere.
For example,
$ cat t.c
struct t {
long c[2];
int d[4];
};
struct t g;
int test()
{
struct t tmp = {.c = {1, 2}, .d = {3, 4}};
g = tmp;
return 0;
}
$ clang -target bpf -O2 -c t.c
$ llvm-readelf -S t.o
...
[ 4] .rodata.cst32 PROGBITS 0000000000000000 0000a8 000020
20 AM 0 0 8
...
In the above code, if you change the struct size, say from 32 bytes to
40 bytes, the rodata.cst32 will go away.
Thanks Yonghong! I guess it is cilium/ebpf needs to recognize rodata.cst32 then
Hi Yonghong,
Here is a follow-up question, it looks cilium/ebpf parse vmlinux and
stores BTF type info in btf.Spec.namedTypes, but the elf object file
provided by user may have section like rodata.cst32 generated by
compiler that does not have accompanying BTF type info stored in
btf.Spec.NamedTypes for the rodata.cst32, how vmlinux can be
guaranteed to have every BTF type info from application/user provided
elf object file ? I guess there is no guarantee.
vmlinux holds kernel types. rodata.cst32 holds data. If the type of
rodata.cst32 needs to be emitted, the type will be encoded in bpf
program BTF.
Did you actually find an issue with .rodata.cst32 section? Such a
section is typically generated by the compiler for initial data
inside the function and llvm bpf backend tries to inline the
values through a bunch of load instructions. So even you see
.rodata.cst32, typically you can safely ignore it.
Vincent
Hi Yonghong,
Thanks for the reproducer. Couldn't figure out what to do with .rodata.cst32,
since there are no symbols and no BTF info for that section.
The values found in .rodata.cst32 are indeed inlined in the bytecode as you
mentioned, so it seems like we can ignore it.
Why does the compiler emit these sections? cilium/ebpf assumed up until now
that all sections starting with '.rodata' are datasecs and must be loaded into
the kernel, which of course needs accompanying BTF.
The clang frontend emits these .rodata.* sections. In early days, kernel
doesn't support global data so llvm bpf backend implements an
optimization to inline these values. But llvm bpf backend didn't completely remove them as the backend doesn't have a global view
whether these .rodata.* are being used in other places or not.
Now, llvm bpf backend has better infrastructure and we probably can
implement an IR pass to detect all uses of .rodata.*, inline these
uses, and remove the .rodata.* global variable.
You can check relocation section of the program text. If the .rodata.*
section is referenced, you should preserve it. Otherwise, you can
ignore that .rodata.* section.
What other .rodata.* should we expect?
Glancing through llvm code, you may see .rodata.{4,8,16,32},
.rodata.str*.
Thanks,
Timo
Thanks for the replies all, very insightful. We were already doing things mostly
right wrt. .rodata.*, but found a few subtle bugs walking through the code again.
I've gotten a hold of the ELF Vincent was trying to load, and I saw a few things
that I found unusual. In his case, the values in cst32 are not inlined. Instead,
this ELF has a .Lconstinit symbol pointing at the start of .rodata.cst32, and it's
an STT_OBJECT with STB_LOCAL. Our relocation handler is fairly strict and requires
STT_OBJECTs to be global (for supporting non-static consts).
There are two ways to resolve the issue. First, extend the loader
support to handle STB_LOCAL as well. Or Second, change the code like
struct t v = {1, 5, 29, ...};
to
struct t v;
__builtin_memset(&v, 0, sizeof(struct t));
v.field1 = ...;
v.field2 = ...;
---
~ llvm-readelf -ar bpf_lxc.o
Symbol table '.symtab' contains 606 entries:
Num: Value Size Type Bind Vis Ndx Name
2: 0000000000000000 32 OBJECT LOCAL DEFAULT 21 .Lconstinit
Relocation section '.rel2/7' at offset 0x6bdf0 contains 173 entries:
Offset Info Type Symbol's Value Symbol's Name
0000000000007300 0000000200000001 R_BPF_64_64 0000000000000000 .Lconstinit
---
---
~ llvm-objdump -S -r -j 2/7 -j .rodata.cst32 bpf_lxc.o
warning: failed to compute relocation: R_BPF_64_64, Invalid data was encountered while parsing the file
... <2 more of these> ...
Disassembly of section 2/7:
00000000000072f8 <LBB1_476>:
3679: 67 08 00 00 03 00 00 00 r8 <<= 3
3680: 18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r2 = 0 ll
0000000000007300: R_BPF_64_64 .Lconstinit
3682: 0f 82 00 00 00 00 00 00 r2 += r8
3683: 79 22 00 00 00 00 00 00 r2 = *(u64 *)(r2 + 0)
3684: 7b 2a 58 ff 00 00 00 00 *(u64 *)(r10 - 168) = r2
Disassembly of section .rodata.cst32:
0000000000000000 <.Lconstinit>:
0: 82 36 4c 98 2e 56 00 00 <unknown>
1: 82 36 4c 98 2e 55 00 00 <unknown>
---
This symbol doesn't exist in the program. Worth noting is that the code that accesses
this static data sits within a subscope, but not sure what the effect of this would be.
Vincent, maybe try removing the enclosing {} to see if that changes anything?
---
static __always_inline int foo(struct __ctx_buff *ctx,
... <snip> ...
{
int i;
for (i = 0; i < VTEP_NUMS; i) {
if (tunnel_endpoint == VTEP_ENDPOINT[i]) {
vtep_mac = VTEP_MAC[i];
break;
}
}
}
---
Is this perhaps something that needs to be addressed in the compiler?
If you can give a reproducible test (with .c or .i file), I can take a
look at what is missing in llvm compiler and improve it.
Thanks again,
Timo
