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![]() Zac reported a verification failure and Alexei reproduced the issue
with a simple reproducer ([1]). The verification failure is due to missed
setting for var_off.
The following is the reproducer in [1]:
0: R1=ctx() R10=fp0
0: (71) r3 = *(u8 *)(r10 -387) ;
R3_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=255,var_off=(0x0; 0xff)) R10=fp0
1: (bc) w7 = (s8)w3 ;
R3_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=255,var_off=(0x0; 0xff))
R7_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=127,var_off=(0x0; 0x7f))
2: (36) if w7 >= 0x2533823b goto pc-3
mark_precise: frame0: last_idx 2 first_idx 0 subseq_idx -1
mark_precise: frame0: regs=r7 stack= before 1: (bc) w7 = (s8)w3
mark_precise: frame0: regs=r3 stack= before 0: (71) r3 = *(u8 *)(r10 -387)
2: R7_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=127,var_off=(0x0; 0x7f))
3: (b4) w0 = 0 ; R0_w=0
4: (95) exit
Note that after insn 1, the var_off for R7 is (0x0; 0x7f). This is not correct
since upper 24 bits of w7 could be 0 or 1. So correct var_off should be
(0x0; 0xffffffff). Missing var_off setting in set_sext32_default_val() caused later
incorrect analysis in zext_32_to_64(dst_reg) and reg_bounds_sync(dst_reg).
To fix the issue, set var_off correctly in set_sext32_default_val(). The correct
reg state after insn 1 becomes:
1: (bc) w7 = (s8)w3 ;
R3_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=255,var_off=(0x0; 0xff))
R7_w=scalar(smin=0,smax=umax=0xffffffff,smin32=-128,smax32=127,var_off=(0x0; 0xffffffff))
and at insn 2, the verifier correctly determines either branch is possible.
[1] https://lore.kernel.org/bpf/CAADnVQLPU0Shz7dWV4bn2BgtGdxN3uFHPeobGBA72tpg5Xoykw@mail.gmail.com/
Fixes:
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.. | ||
preload | ||
arena.c | ||
arraymap.c | ||
bloom_filter.c | ||
bpf_cgrp_storage.c | ||
bpf_inode_storage.c | ||
bpf_iter.c | ||
bpf_local_storage.c | ||
bpf_lru_list.c | ||
bpf_lru_list.h | ||
bpf_lsm.c | ||
bpf_struct_ops.c | ||
bpf_task_storage.c | ||
btf.c | ||
cgroup.c | ||
cgroup_iter.c | ||
core.c | ||
cpumap.c | ||
cpumask.c | ||
crypto.c | ||
devmap.c | ||
disasm.c | ||
disasm.h | ||
dispatcher.c | ||
hashtab.c | ||
helpers.c | ||
inode.c | ||
Kconfig | ||
link_iter.c | ||
local_storage.c | ||
log.c | ||
lpm_trie.c | ||
Makefile | ||
map_in_map.c | ||
map_in_map.h | ||
map_iter.c | ||
memalloc.c | ||
mmap_unlock_work.h | ||
mprog.c | ||
net_namespace.c | ||
offload.c | ||
percpu_freelist.c | ||
percpu_freelist.h | ||
prog_iter.c | ||
queue_stack_maps.c | ||
reuseport_array.c | ||
ringbuf.c | ||
stackmap.c | ||
syscall.c | ||
sysfs_btf.c | ||
task_iter.c | ||
tcx.c | ||
tnum.c | ||
token.c | ||
trampoline.c | ||
verifier.c |