Re: [PATCH] objtool: Fix infinite loop in for_offset_range()

From: Randy Dunlap
Date: Sat Apr 25 2020 - 16:08:10 EST


On 4/25/20 1:04 PM, Randy Dunlap wrote:
> On 4/25/20 12:19 PM, Josh Poimboeuf wrote:
>> Randy reported that objtool got stuck in an infinite loop when
>> processing drivers/i2c/busses/i2c-parport.o. It was caused by the
>> following code:
>>
>> 00000000000001fd <line_set>:
>> 1fd: 48 b8 00 00 00 00 00 movabs $0x0,%rax
>> 204: 00 00 00
>> 1ff: R_X86_64_64 .rodata-0x8
>> 207: 41 55 push %r13
>> 209: 41 89 f5 mov %esi,%r13d
>> 20c: 41 54 push %r12
>> 20e: 49 89 fc mov %rdi,%r12
>> 211: 55 push %rbp
>> 212: 48 89 d5 mov %rdx,%rbp
>> 215: 53 push %rbx
>> 216: 0f b6 5a 01 movzbl 0x1(%rdx),%ebx
>> 21a: 48 8d 34 dd 00 00 00 lea 0x0(,%rbx,8),%rsi
>> 221: 00
>> 21e: R_X86_64_32S .rodata
>> 222: 48 89 f1 mov %rsi,%rcx
>> 225: 48 29 c1 sub %rax,%rcx
>>
>> find_jump_table() saw the .rodata reference and tried to find a jump
>> table associated with it (though there wasn't one). The -0x8 rela
>> addend is unusual. It caused find_jump_table() to send a negative
>> table_offset (unsigned 0xfffffffffffffff8) to find_rela_by_dest().
>>
>> The negative offset should have been harmless, but it actually threw
>> for_offset_range() for a loop... literally. When the mask value got
>> incremented past the end value, it also wrapped to zero, causing the
>> loop exit condition to remain true forever.
>>
>> Prevent this scenario from happening by ensuring the incremented value
>> is always >= the starting value.
>>
>> Fixes: 74b873e49d92 ("objtool: Optimize find_rela_by_dest_range()")
>> Reported-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
>> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
>
> Hi Josh,
>
> When applied to linux-next 20200414 (where it was reported) and using
> config-r2092, objtool still loops (I killed it after 6 minutes of CPU time).

Nope, scratch that. Operator error. Sorry.

> When applied to linux-next 20200424 and using the same config-r2092 file,
> objtool appears to terminate normally and the entire build does also.
>
> Acked-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Tested-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
>
> thanks.
>
>> ---


--
~Randy