cdfs crashes when trying to mount HelenOS boot CD

[Jiri Svoboda]

With latest mainline and amd64 when I try mounting the boot CD, cdfs crashes (and the command line hangs).

I tried the same with revisions as old as 2408 (December 2015) where it had to work (because sysinst is introduced later) with the same result. That means it cannot be a regression in HelenOS. My current tool that is called to create the ISO image (Fedora genisoimage called as mkisofs) must have started doing something different that cdfs doesn't like.

[Jiri Svoboda]

Mounting the boot CD

[Jiri Svoboda]

Stack trace (kcon)

[Jiri Svoboda]

The screen shots are from revision 2408. In current rev 2688, the crash occurs in cdfs_readdir() on address 1b76:

                        buf[i] = uint16_t_be2host(((uint16_t *)data)[i]);
    1b5e:       66 0f ef d2             pxor   %xmm2,%xmm2
    1b62:       48 29 c7                sub    %rax,%rdi
    1b65:       48 01 c0                add    %rax,%rax
    1b68:       4d 8d 04 06             lea    (%r14,%rax,1),%r8
    1b6c:       48 89 fe                mov    %rdi,%rsi
    1b6f:       4c 01 e8                add    %r13,%rax
    1b72:       48 c1 ee 03             shr    $0x3,%rsi
    1b76:       66 41 0f 6f 08          movdqa (%r8),%xmm1
    1b7b:       48 83 fe 01             cmp    $0x1,%rsi
    1b7f:       66 0f 6f c1             movdqa %xmm1,%xmm0
    1b83:       66 0f 6f d9             movdqa %xmm1,%xmm3

[Jiri Svoboda]

Note that this is a general protection fault, not a page fault. Note that the faulting instruction is movdqa (move aligned double quadword). Looks like the problem is that data is at an odd address, but we're reading it as a 16-bit value. The compiler generated movdqa instruction which generates GP if the adress is not actually aligned.

[Jiri Svoboda]

The problem was we were trying to read the UCS2 file names word-by-word (in the C source), but they were starting at an odd address. Interestingly enough, the compiler generates SSE instructions and reads the string in units of 128 bits (16 bytes). I didn't look into this in detail, but it seems the code is generated so that it deals with any even address, but not with odd addresses.

[Jiri Svoboda]

Fixed in changeset:mainline,2689 using unaligned_uint16_t * from unaligned.h.