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s390/bpf: Tolerate not converging code shrinking

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"BPF_MAXINSNS: Maximum possible literals" unnecessarily falls back to
the interpreter because of failing sanity check in bpf_set_addr. The
problem is that there are a lot of branches that can be shrunk, and
doing so opens up the possibility to shrink even more. This process
does not converge after 3 passes, causing code offsets to change during
the codegen pass, which must never happen.

Fix by inserting nops during codegen pass in order to preserve code
offets.

Fixes: 4e9b4a6883 ("s390/bpf: Use relative long branches")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200717165326.6786-5-iii@linux.ibm.com
This commit is contained in:
Ilya Leoshkevich 2020-07-17 18:53:25 +02:00 committed by Alexei Starovoitov
parent 5fa6974471
commit 1491b73311
1 changed files with 26 additions and 1 deletions
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27
arch/s390/net/bpf_jit_comp.c
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@ -489,6 +489,24 @@ static void save_restore_regs(struct bpf_jit *jit, int op, u32 stack_depth)
} while (re <= last);
}
static void bpf_skip(struct bpf_jit *jit, int size)
{
if (size >= 6 && !is_valid_rel(size)) {
/* brcl 0xf,size */
EMIT6_PCREL_RIL(0xc0f4000000, size);
size -= 6;
} else if (size >= 4 && is_valid_rel(size)) {
/* brc 0xf,size */
EMIT4_PCREL(0xa7f40000, size);
size -= 4;
}
while (size >= 2) {
/* bcr 0,%0 */
_EMIT2(0x0700);
size -= 2;
}
}
/*
* Emit function prologue
*
@ -1503,7 +1521,14 @@ static bool bpf_is_new_addr_sane(struct bpf_jit *jit, int i)
*/
static int bpf_set_addr(struct bpf_jit *jit, int i)
{
if (!bpf_is_new_addr_sane(jit, i))
int delta;
if (is_codegen_pass(jit)) {
delta = jit->prg - jit->addrs[i];
if (delta < 0)
bpf_skip(jit, -delta);
}
if (WARN_ON_ONCE(!bpf_is_new_addr_sane(jit, i)))
return -1;
jit->addrs[i] = jit->prg;
return 0;