Re: [PATCH 08/19] vdso/gettimeofday: Prepare do_hres_timens() for introduction of struct vdso_clock

[Jan Stancek <jstancek@redhat.com>]

On Fri, Apr 25, 2025 at 03:40:55PM +0200, Thomas Weißschuh wrote:

<snip>

>
>Some more information:
>
>The crash comes from the address arithmetic in "vc = &vc[CS_RAW]" going wrong.

That appears to be because it's not doing any arithmetic, but using value
from some linker-generated symbol (I'll refer to it as "7a8").

The below is presumably the check that compares clk != CLOCK_MONOTONIC_RAW,
CLOCK_MONOTONIC_RAW is 4. And it should choose between first and second vdso_clock
in 2nd vvar page:

# readelf -a /root/kernel-ark/arch/arm64/kernel/vdso/vdso.so.dbg | grep 7a8
     25: 00000000000007a8     0 NOTYPE  LOCAL  DEFAULT    7 $d

objdump -d -S:
  658:   17ffffef        b       614 <__cvdso_clock_gettime_data.constprop.0+0x104>
                 vc = &vc[CS_HRES_COARSE];
  65c:   58000a62        ldr     x2, 7a8 <__cvdso_clock_gettime_data.constprop.0+0x298>
  660:   7100101f        cmp     w0, #0x4
  664:   10e7cce3        adr     x3, fffffffffffd0000 <vdso_u_data+0x10000>
  668:   9a821063        csel    x3, x3, x2, ne  // ne = any
         while (unlikely((seq = READ_ONCE(vc->seq)) & 1))
  66c:   b9400065        ldr     w5, [x3]
...
  7a8:   fffd00e0        .word   0xfffd00e0
  7ac:   ffffffff        .word   0xffffffff


(gdb) r
Thread 2.1 "a.out" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xfffff7ff2dc0 (LWP 44638)]
0x0000fffff7fa066c in ?? ()

(gdb) disassemble 0x0000fffff7fa0658,0x0000fffff7fa067c
--Type <RET> for more, q to quit, c to continue without paging--
    0x0000fffff7fa0658:  b       0xfffff7fa0614
    0x0000fffff7fa065c:  ldr     x2, 0xfffff7fa07a8
    0x0000fffff7fa0660:  cmp     w0, #0x4
    0x0000fffff7fa0664:  adr     x3, 0xfffff7f70000
    0x0000fffff7fa0668:  csel    x3, x3, x2, ne  // ne = any
=> 0x0000fffff7fa066c:  ldr     w5, [x3]

$x3 here is using correct vvar runtime address, but $x2 uses
a bogus one from "7a8". And when it tries to load from it, it crashes:

(gdb) info proc map
process 44638
Mapped address spaces:

Start Addr         End Addr           Size               Offset             Perms File
0x0000000000400000 0x0000000000410000 0x10000            0x0                r-xp  /root/ltp/testcases/kernel/syscalls/clock_gettime/a.out
0x0000000000410000 0x0000000000420000 0x10000            0x0                r--p  /root/ltp/testcases/kernel/syscalls/clock_gettime/a.out
0x0000000000420000 0x0000000000430000 0x10000            0x10000            rw-p  /root/ltp/testcases/kernel/syscalls/clock_gettime/a.out
0x0000fffff7d80000 0x0000fffff7f30000 0x1b0000           0x0                r-xp  /usr/lib64/libc.so.6
0x0000fffff7f30000 0x0000fffff7f40000 0x10000            0x1a0000           r--p  /usr/lib64/libc.so.6
0x0000fffff7f40000 0x0000fffff7f50000 0x10000            0x1b0000           rw-p  /usr/lib64/libc.so.6
0x0000fffff7f60000 0x0000fffff7fa0000 0x40000            0x0                r--p  [vvar]
0x0000fffff7fa0000 0x0000fffff7fb0000 0x10000            0x0                r-xp  [vdso]
0x0000fffff7fb0000 0x0000fffff7fe0000 0x30000            0x0                r-xp  /usr/lib/ld-linux-aarch64.so.1
0x0000fffff7fe0000 0x0000fffff7ff0000 0x10000            0x20000            r--p  /usr/lib/ld-linux-aarch64.so.1
0x0000fffff7ff0000 0x0000fffff8000000 0x10000            0x30000            rw-p  /usr/lib/ld-linux-aarch64.so.1
0x0000fffffffd0000 0x0001000000000000 0x30000            0x0                rw-p  [stack]

$x3 was the beginning of 2nd vvar page (1st vdso_clock), and $x2 looks
like it should have been 2nd vdso_clock, but the value that is used is not
the address of vvar during runtime. 

vdso_clock has size 224 == 0xe0 (according to pahole)

(gdb) x/2x 0xfffff7fa07a8
0xfffff7fa07a8: 0xfffd00e0      0xffffffff
(gdb) p/x $x2
$1 = 0xfffffffffffd00e0
(gdb) x/1x $x2
0xfffffffffffd00e0:     Cannot access memory at address 0xfffffffffffd00e0

But it does match the symbol value from vdso.so:

# readelf -a /root/kernel-ark/arch/arm64/kernel/vdso/vdso.so.dbg | grep vdso_u
     37: fffffffffffc0000     0 NOTYPE  LOCAL  DEFAULT    1 vdso_u_data
     39: fffffffffffc0000     0 NOTYPE  LOCAL  DEFAULT  ABS vdso_u_time_data
     40: fffffffffffe0000     0 NOTYPE  LOCAL  DEFAULT  ABS vdso_u_rng_data

>>> print(hex(0xfffffffffffc0000+65536+224))
0xfffffffffffd00e0 -> and this is address where it crashes

Regards,
Jan

>This should just do "vc = vc + 1", advancing the pointer by sizeof(*vc).
>But I get these example values of "vc" before and after the arithmetic:
>0x00ffffbc060000 -> 0xfffffffffffd00e8
>Which is obviously wrong.
>
>The arithmetic can be fixed with any one of the following changes:
>* OPTIMIZER_HIDE_VAR() as above
>* Replacement of -mcmodel=tiny with -mcmode=small in arch/arm64/kernel/vdso/Makefile
>  -mcmodel=tiny is supposed to cover 1MiB programs, the vDSO only needs 300KiB here
>* Removal of the (clk != CLOCK_MONOTONIC_RAW) case
>
>Shuffling code around, even without any impact on semantics, sometimes fixes
>the issue. Compiling the vDSO with UBSAN didn't show anything.
>
>
>Thomas
>
>> > > > > [1] https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/syscalls/clock_gettime/clock_gettime03.c
>> > > > > [2] https://koji.fedoraproject.org/koji/buildinfo?buildID=2704401
>> > > >
>> >
>>
>