Last night both Firefox and Chrome began crashing at startup. After fighting with it for almost an hour I gave up and kept working, resolving to fix them in the morning.

Well, then Vim started crashing at startup too. Uh uh honey, no no no.

A tail of dmesg revealed the problem - libm-2.19:

[10567.643135] traps: python[18389] general protection ip:7efe0933eea2 sp:7ffff5087f30 error:0 in[7efe0932a000+105000]
[10591.556424] traps: chromium-browse[18584] general protection ip:7f6aaed27ea2 sp:7ffc2a01cf30 error:0 in[7f6aaed13000+105000]
[10875.658269] traps: apt-check[18686] general protection ip:7fd1d03f4ea2 sp:7ffdc7554530 error:0 in[7fd1d03e0000+105000]

I replaced libm-2.19 with a version from another machine running the same distro and version, and everything started working again.

But, having spent a lot of time raging and slapping down commands, I really wanted to know what the hell happened to libm.

Introducing dhex, a hex editor with an awesome diff mode.

That is it. Somehow an 0xC1 became an 0xE1. Yep, 0x1100 0001 became 0x1110 0001.

One bad bit, one bad byte, one bad lib.

Imagine the mayhem one bad bit in libc could cause.


If you are curious what instructions the bit flip changed, here it is:

diff <(objdump -D -M intel | grep -E '^\s+14e..:') \
     <(objdump -D -M intel | grep -E '^\s+14e..:')

<    14e9e:	c1 fa 14             	sar    edx,0x14
<    14ea1:	81 e6 ff ff 0f 00    	and    esi,0xfffff
<    14ea7:	81 fe 9d a0 06 00    	cmp    esi,0x6a09d
<    14ead:	0f 8f 4d 04 00 00    	jg     15300 <__log10_finite+0x680>
<    14eb3:	89 d1                	mov    ecx,edx
<    14eb5:	81 ce 00 00 f0 3f    	or     esi,0x3ff00000
>    14e9e:	e1 fa                	loope  14e9a <__log10_finite+0x21a>
>    14ea0:	14 81                	adc    al,0x81
>    14ea2:	e6 ff                	out    0xff,al
>    14ea4:	ff 0f                	dec    DWORD PTR [rdi]
>    14ea6:	00 81 fe 9d a0 06    	add    BYTE PTR [rcx+0x6a09dfe],al
>    14eac:	00 0f                	add    BYTE PTR [rdi],cl
>    14eae:	8f                   	(bad)
>    14eaf:	4d 04 00             	rex.WRB add al,0x0
>    14eb2:	00 89 d1 81 ce 00    	add    BYTE PTR [rcx+0xce81d1],cl
>    14eb8:	00 f0                	add    al,dh
>    14eba:	3f                   	(bad)

Looks like I need to start backing up the disk, now replace the power supply RAM.