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s390/bpf: Add s390 JIT support for timed may_goto

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The verifier provides an architecture-independent implementation of the
may_goto instruction, which is currently used on s390x, but it has a
downside: there is no way to prevent progs using it from running for a
very long time.

The solution to this problem is an alternative timed implementation,
which requires architecture-specific bits. Its availability is signaled
to the verifier by bpf_jit_supports_timed_may_goto() returning true.

The verifier then emits a call to arch_bpf_timed_may_goto() using a
non-standard calling convention. This function must act as a trampoline
for bpf_check_timed_may_goto().

Implement bpf_jit_supports_timed_may_goto(), account for the special
calling convention in the BPF_CALL implementation, and implement
arch_bpf_timed_may_goto().

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20250821113339.292434-2-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Ilya Leoshkevich
2025-08-21 13:25:55 +02:00
committed by Alexei Starovoitov
parent d0f27ff27c
commit b8efa810c1
3 changed files with 67 additions and 5 deletions
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2
arch/s390/net/Makefile
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@@ -2,5 +2,5 @@
#
# Arch-specific network modules
#
obj-$(CONFIG_BPF_JIT) += bpf_jit_comp.o
obj-$(CONFIG_BPF_JIT) += bpf_jit_comp.o bpf_timed_may_goto.o
obj-$(CONFIG_HAVE_PNETID) += pnet.o

25
arch/s390/net/bpf_jit_comp.c
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@@ -1806,10 +1806,22 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
}
}
/* brasl %r14,func */
EMIT6_PCREL_RILB_PTR(0xc0050000, REG_14, (void *)func);
/* lgr %b0,%r2: load return value into %b0 */
EMIT4(0xb9040000, BPF_REG_0, REG_2);
if ((void *)func == arch_bpf_timed_may_goto) {
/*
* arch_bpf_timed_may_goto() has a special ABI: the
* parameters are in BPF_REG_AX and BPF_REG_10; the
* return value is in BPF_REG_AX; and all GPRs except
* REG_W0, REG_W1, and BPF_REG_AX are callee-saved.
*/
/* brasl %r0,func */
EMIT6_PCREL_RILB_PTR(0xc0050000, REG_0, (void *)func);
} else {
/* brasl %r14,func */
EMIT6_PCREL_RILB_PTR(0xc0050000, REG_14, (void *)func);
/* lgr %b0,%r2: load return value into %b0 */
EMIT4(0xb9040000, BPF_REG_0, REG_2);
}
/*
* Copy the potentially updated tail call counter back.
@@ -2993,3 +3005,8 @@ void arch_bpf_stack_walk(bool (*consume_fn)(void *, u64, u64, u64),
prev_addr = addr;
}
}
bool bpf_jit_supports_timed_may_goto(void)
{
return true;
}

45
arch/s390/net/bpf_timed_may_goto.S Normal file
View File

@@ -0,0 +1,45 @@
/* SPDX-License-Identifier: GPL-2.0 */
#include <linux/export.h>
#include <linux/linkage.h>
#include <asm/asm-offsets.h>
#include <asm/nospec-insn.h>
#define R2_OFF 0
#define R5_OFF (R2_OFF + (5 - 2 + 1) * 8)
#define R14_OFF (R5_OFF + 8)
#define RETADDR_OFF (R14_OFF + 8)
#define R15_OFF (RETADDR_OFF + 8)
#define BACKCHAIN_OFF (R15_OFF + 8)
#define FRAME_SIZE (BACKCHAIN_OFF + 8)
#define FRAME_OFF (STACK_FRAME_OVERHEAD - FRAME_SIZE)
#if (FRAME_OFF + BACKCHAIN_OFF) != __SF_BACKCHAIN
#error Stack frame layout calculation is broken
#endif
GEN_BR_THUNK %r1
SYM_FUNC_START(arch_bpf_timed_may_goto)
/*
* This function has a special ABI: the parameters are in %r12 and
* %r13; the return value is in %r12; all GPRs except %r0, %r1, and
* %r12 are callee-saved; and the return address is in %r0.
*/
stmg %r2,%r5,FRAME_OFF+R2_OFF(%r15)
stg %r14,FRAME_OFF+R14_OFF(%r15)
stg %r0,FRAME_OFF+RETADDR_OFF(%r15)
stg %r15,FRAME_OFF+R15_OFF(%r15)
lgr %r1,%r15
lay %r15,-FRAME_SIZE(%r15)
stg %r1,__SF_BACKCHAIN(%r15)
lay %r2,0(%r12,%r13)
brasl %r14,bpf_check_timed_may_goto
lgr %r12,%r2
lg %r15,FRAME_SIZE+FRAME_OFF+R15_OFF(%r15)
lmg %r2,%r5,FRAME_OFF+R2_OFF(%r15)
lg %r14,FRAME_OFF+R14_OFF(%r15)
lg %r1,FRAME_OFF+RETADDR_OFF(%r15)
BR_EX %r1
SYM_FUNC_END(arch_bpf_timed_may_goto)