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bpf: x86: Support in-register struct arguments in trampoline programs

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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@fb.com>
Link: https://lore.kernel.org/r/20220831152652.2078600-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Yonghong Song 2022-08-31 08:26:52 -07:00 committed by Alexei Starovoitov
parent 720e6a4351
commit a9c5ad31fb
1 changed files with 51 additions and 17 deletions
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68
arch/x86/net/bpf_jit_comp.c
View File

@ -1751,34 +1751,60 @@ emit_jmp:
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];
}
while (nr_regs) {
emit_stx(prog, bytes_to_bpf_size(arg_size),
BPF_REG_FP,
j == 5 ? X86_REG_R9 : BPF_REG_1 + j,
-(stack_size - j * 8));
nr_regs--;
j++;
}
}
}
static void restore_regs(const struct btf_func_model *m, u8 **prog, int nr_args,
int stack_size)
{
int i;
int i, j, arg_size, nr_regs;
/* Restore function arguments from stack.
* For a function that accepts two pointers the sequence will be:
* EMIT4(0x48, 0x8B, 0x7D, 0xF0); mov rdi,QWORD PTR [rbp-0x10]
* EMIT4(0x48, 0x8B, 0x75, 0xF8); mov rsi,QWORD PTR [rbp-0x8]
*/
for (i = 0; i < min(nr_args, 6); i++)
emit_ldx(prog, bytes_to_bpf_size(m->arg_size[i]),
i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
BPF_REG_FP,
-(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];
}
while (nr_regs) {
emit_ldx(prog, bytes_to_bpf_size(arg_size),
j == 5 ? X86_REG_R9 : BPF_REG_1 + j,
BPF_REG_FP,
-(stack_size - j * 8));
nr_regs--;
j++;
}
}
}
static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,
@ -2015,7 +2041,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
struct bpf_tramp_links *tlinks,
void *orig_call)
{
int ret, i, nr_args = m->nr_args;
int ret, i, nr_args = m->nr_args, extra_nregs = 0;
int regs_off, ip_off, args_off, stack_size = nr_args * 8, run_ctx_off;
struct bpf_tramp_links *fentry = &tlinks[BPF_TRAMP_FENTRY];
struct bpf_tramp_links *fexit = &tlinks[BPF_TRAMP_FEXIT];
@ -2028,6 +2054,14 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
if (nr_args > 6)
return -ENOTSUPP;
for (i = 0; i < MAX_BPF_FUNC_ARGS; i++) {
if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG)
extra_nregs += (m->arg_size[i] + 7) / 8 - 1;
}
if (nr_args + extra_nregs > 6)
return -ENOTSUPP;
stack_size += extra_nregs * 8;
/* Generated trampoline stack layout:
*
* RBP + 8 [ return address ]
@ -2040,7 +2074,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
* [ ... ]
* RBP - regs_off [ reg_arg1 ] program's ctx pointer
*
* RBP - args_off [ args count ] always
* RBP - args_off [ arg regs count ] always
*
* RBP - ip_off [ traced function ] BPF_TRAMP_F_IP_ARG flag
*
@ -2083,11 +2117,11 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
EMIT4(0x48, 0x83, 0xEC, stack_size); /* sub rsp, stack_size */
EMIT1(0x53); /* push rbx */
/* Store number of arguments of the traced function:
* mov rax, nr_args
/* Store number of argument registers of the traced function:
* mov rax, nr_args + extra_nregs
* mov QWORD PTR [rbp - args_off], rax
*/
emit_mov_imm64(&prog, BPF_REG_0, 0, (u32) nr_args);
emit_mov_imm64(&prog, BPF_REG_0, 0, (u32) nr_args + extra_nregs);
emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -args_off);
if (flags & BPF_TRAMP_F_IP_ARG) {