The main motivation for private stack comes from nested scheduler in
sched-ext from Tejun. The basic idea is that
- each cgroup will its own associated bpf program,
- bpf program with parent cgroup will call bpf programs
in immediate child cgroups.
Let us say we have the following cgroup hierarchy:
root_cg (prog0):
cg1 (prog1):
cg11 (prog11):
cg111 (prog111)
cg112 (prog112)
cg12 (prog12):
cg121 (prog121)
cg122 (prog122)
cg2 (prog2):
cg21 (prog21)
cg22 (prog22)
cg23 (prog23)
In the above example, prog0 will call a kfunc which will call prog1 and
prog2 to get sched info for cg1 and cg2 and then the information is
summarized and sent back to prog0. Similarly, prog11 and prog12 will be
invoked in the kfunc and the result will be summarized and sent back to
prog1, etc.
Currently, for each thread, the x86 kernel allocate 8KB stack. The each
bpf program (including its subprograms) has maximum 512B stack size to
avoid potential stack overflow. And nested bpf programs increase the risk
of stack overflow. To avoid potential stack overflow caused by bpf
programs, this patch implemented a private stack so bpf program stack
space is allocated dynamically when the program is jited. Such private
stack is applied to tracing programs like kprobe/uprobe, perf_event,
tracepoint, raw tracepoint and tracing.
But more than one instance of the same bpf program may run in the system.
To make things simple, percpu private stack is allocated for each program,
so if the same program is running on different cpus concurrently, we won't
have any issue. Note that the kernel already have logic to prevent the
recursion for the same bpf program on the same cpu (kprobe, fentry, etc.).
The patch implemented a percpu private stack based approach for x86 arch.
A new kfunc bpf_prog_call() is introduced for the above nested scheduler use
case. If bpf_prog_call() is used in the program and bpf_tail_call() is not
used in the same program, then private stack will be used. Internally,
private stack allows certain number of recursions by allocating more
space. Please see each individual patch for details.
Change logs:
v3 -> v4:
- v3 link: https://lore.kernel.org/bpf/20240926234506.1769256-1-yonghong.song@xxxxxxxxx/
There is a long discussion in the above v3 link trying to allow private
stack to be used by kernel functions in order to simplify implementation.
But unfortunately we didn't find a workable solution yet, so we return
to the approach where private stack is only used by bpf programs.
- Add bpf_prog_call() kfunc.
v2 -> v3:
- Instead of per-subprog private stack allocation, allocate private
stacks at main prog or callback entry prog. Subprogs not main or callback
progs will increment the inherited stack pointer to be their
frame pointer.
- Private stack allows each prog max stack size to be 512 bytes, intead
of the whole prog hierarchy to be 512 bytes.
- Add some tests.
Yonghong Song (10):
bpf: Allow each subprog having stack size of 512 bytes
bpf: Mark each subprog with proper private stack modes
bpf, x86: Refactor func emit_prologue
bpf, x86: Create a helper for certain "reg <op>= imm" operations
bpf, x86: Add jit support for private stack
selftests/bpf: Add private stack tests
bpf: Support calling non-tailcall bpf prog
bpf, x86: Create two helpers for some arith operations
bpf, x86: Jit support for nested bpf_prog_call
selftests/bpf: Add tests for bpf_prog_call()
arch/x86/net/bpf_jit_comp.c | 318 ++++++++++++++----
include/linux/bpf.h | 14 +
include/linux/bpf_verifier.h | 3 +
include/linux/filter.h | 1 +
kernel/bpf/core.c | 27 ++
kernel/bpf/helpers.c | 20 ++
kernel/bpf/trampoline.c | 16 +
kernel/bpf/verifier.c | 145 +++++++-
.../selftests/bpf/prog_tests/prog_call.c | 78 +++++
.../selftests/bpf/prog_tests/verifier.c | 2 +
tools/testing/selftests/bpf/progs/prog_call.c | 92 +++++
.../bpf/progs/verifier_private_stack.c | 216 ++++++++++++
12 files changed, 856 insertions(+), 76 deletions(-)
create mode 100644 tools/testing/selftests/bpf/prog_tests/prog_call.c
create mode 100644 tools/testing/selftests/bpf/progs/prog_call.c
create mode 100644 tools/testing/selftests/bpf/progs/verifier_private_stack.c
--
2.43.5
