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bpf: Fix abs(INT_MIN) undefined behavior in interpreter sdiv/smod #11343

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bpf: Fix abs(INT_MIN) undefined behavior in interpreter sdiv/smod #11343

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58e2940
bpf: Fix undefined behavior in interpreter sdiv/smod for INT_MIN
Mar 9, 2026
2736632
selftests/bpf: Add tests for sdiv32/smod32 with INT_MIN dividend
Mar 9, 2026
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22 changes: 14 additions & 8 deletions kernel/bpf/core.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1736,6 +1736,12 @@ bool bpf_opcode_in_insntable(u8 code)
}

#ifndef CONFIG_BPF_JIT_ALWAYS_ON
/* Safe absolute value for s32 to prevent undefined behavior for abs(S32_MIN) */
static inline u32 __safe_abs32(s32 x)
{
return x >= 0 ? (u32)x : -(u32)x;
}

/**
* ___bpf_prog_run - run eBPF program on a given context
* @regs: is the array of MAX_BPF_EXT_REG eBPF pseudo-registers
Expand Down Expand Up @@ -1900,8 +1906,8 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
DST = do_div(AX, (u32) SRC);
break;
case 1:
AX = abs((s32)DST);
AX = do_div(AX, abs((s32)SRC));
AX = __safe_abs32((s32)DST);
AX = do_div(AX, __safe_abs32((s32)SRC));
if ((s32)DST < 0)
DST = (u32)-AX;
else
Expand All @@ -1928,8 +1934,8 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
DST = do_div(AX, (u32) IMM);
break;
case 1:
AX = abs((s32)DST);
AX = do_div(AX, abs((s32)IMM));
AX = __safe_abs32((s32)DST);
AX = do_div(AX, __safe_abs32((s32)IMM));
if ((s32)DST < 0)
DST = (u32)-AX;
else
Expand All @@ -1955,8 +1961,8 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
DST = (u32) AX;
break;
case 1:
AX = abs((s32)DST);
do_div(AX, abs((s32)SRC));
AX = __safe_abs32((s32)DST);
do_div(AX, __safe_abs32((s32)SRC));
if (((s32)DST < 0) == ((s32)SRC < 0))
DST = (u32)AX;
else
Expand All @@ -1982,8 +1988,8 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
DST = (u32) AX;
break;
case 1:
AX = abs((s32)DST);
do_div(AX, abs((s32)IMM));
AX = __safe_abs32((s32)DST);
do_div(AX, __safe_abs32((s32)IMM));
if (((s32)DST < 0) == ((s32)IMM < 0))
DST = (u32)AX;
else
Expand Down
58 changes: 58 additions & 0 deletions tools/testing/selftests/bpf/progs/verifier_sdiv.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1209,6 +1209,64 @@ __naked void smod32_ri_divisor_neg_1(void)
: __clobber_all);
}

SEC("socket")
__description("SDIV32, INT_MIN divided by 2, imm")
__success __success_unpriv __retval(-1073741824)
__naked void sdiv32_int_min_div_2_imm(void)
{
asm volatile (" \
w0 = %[int_min]; \
w0 s/= 2; \
exit; \
" :
: __imm_const(int_min, INT_MIN)
: __clobber_all);
}

SEC("socket")
__description("SDIV32, INT_MIN divided by 2, reg")
__success __success_unpriv __retval(-1073741824)
__naked void sdiv32_int_min_div_2_reg(void)
{
asm volatile (" \
w0 = %[int_min]; \
w1 = 2; \
w0 s/= w1; \
exit; \
" :
: __imm_const(int_min, INT_MIN)
: __clobber_all);
}

SEC("socket")
__description("SMOD32, INT_MIN modulo 2, imm")
__success __success_unpriv __retval(0)
__naked void smod32_int_min_mod_2_imm(void)
{
asm volatile (" \
w0 = %[int_min]; \
w0 s%%= 2; \
exit; \
" :
: __imm_const(int_min, INT_MIN)
: __clobber_all);
}

SEC("socket")
__description("SMOD32, INT_MIN modulo -2, imm")
__success __success_unpriv __retval(0)
__naked void smod32_int_min_mod_neg2_imm(void)
{
asm volatile (" \
w0 = %[int_min]; \
w0 s%%= -2; \
exit; \
" :
: __imm_const(int_min, INT_MIN)
: __clobber_all);
}


#else

SEC("socket")
Expand Down
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