On 9/6/22 8:00 PM, Alexei Starovoitov wrote:
On Wed, Aug 31, 2022 at 8:26 AM Yonghong Song <yhs@xxxxxx> wrote:
In C, struct value can be passed as a function argument.
For small structs, struct value may be passed in
one or more registers. For trampoline based bpf programs,
this would cause complication since one-to-one mapping between
function argument and arch argument register is not valid
any more.
The latest llvm16 added bpf support to pass by values
for struct up to 16 bytes ([1]). This is also true for
x86_64 architecture where two registers will hold
the struct value if the struct size is >8 and <= 16.
This may not be true if one of struct member is 'double'
type but in current linux source code we don't have
such instance yet, so we assume all >8 && <= 16 struct
holds two general purpose argument registers.
Also change on-stack nr_args value to the number
of registers holding the arguments. This will
permit bpf_get_func_arg() helper to get all
argument values.
[1] https://reviews.llvm.org/D132144
Signed-off-by: Yonghong Song <yhs@xxxxxx>
---
arch/x86/net/bpf_jit_comp.c | 68 +++++++++++++++++++++++++++----------
1 file changed, 51 insertions(+), 17 deletions(-)
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index c1f6c1c51d99..ae89f4143eb4 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1751,34 +1751,60 @@ st: if (is_imm8(insn->off))
static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_args,
int stack_size)
{
- int i;
+ int i, j, arg_size, nr_regs;
/* Store function arguments to stack.
* For a function that accepts two pointers the sequence will be:
* mov QWORD PTR [rbp-0x10],rdi
* mov QWORD PTR [rbp-0x8],rsi
*/
- for (i = 0; i < min(nr_args, 6); i++)
- emit_stx(prog, bytes_to_bpf_size(m->arg_size[i]),
- BPF_REG_FP,
- i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
- -(stack_size - i * 8));
+ for (i = 0, j = 0; i < min(nr_args, 6); i++) {
+ if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG) {
+ nr_regs = (m->arg_size[i] + 7) / 8;
+ arg_size = 8;
+ } else {
+ nr_regs = 1;
+ arg_size = m->arg_size[i];
+ }
This bit begs for a common helper, but I'm not sure
whether it will look better, so applied as-is.
BPF_PROG2 also feels unusual as an API macro name.
We probably should bikeshed a bit and follow up
if a better name is found.
I didn't come up with a better name either. But happy
to change to a different name if we agreed on one.
