snap run --strace can't find setpriority on (at least 4.4)

[Jamie Strandboge]

In investigating a different issue, I ran this on bionic and newer:

$ sudo snap install test-snapd-tools
$ sudo snap run --strace test-snapd-tools.cmd nice -n -10 uptime 2>&1 | grep setpriority
[pid 28394] setpriority(PRIO_PROCESS, 0, -10) = -1 EPERM (Operation not permitted)

which is the expected output.

# xenial

I then tried on xenial and got different behavior:

$ sudo snap install test-snapd-tools
$ sudo snap run --strace test-snapd-tools.cmd nice -n -10 uptime 2>&1 | grep setpriority
$

Ie, strace didn't report the setpriority syscall. I then looked at the full strace and found that strace couldn't find setpriority (strace will prefix syscall_ with a hex value when it can't find the syscall, as per its man page):

[pid 5770] syscall_18446744073709551615(0, 0, 0xfffffff6, 0x865, 0, 0x1999999999999999) = -1 (errno 1)

Note, this 18446744073709551615 seems an odd value for the syscall...

If I use the old method of stracing described in https://forum.snapcraft.io/t/stracing-snap-commands/1433, the syscall is also not found:

$ sudo strace -e '!select,pselect6,_newselect,clock_gettime' -f -D -vv test-snapd-tools.cmd nice -n -10 uptime 2>&1 | grep syscall_
[pid 5875] syscall_18446744073709551615(0, 0, 0xfffffff6, 0x865, 0, 0x1999999999999999) = -1 (errno 1)

However, it does if I just call strace directly on the command outside of snap run:

$ sudo strace -- nice -n -10 uptime 2>&1 | grep -E '(syscall_|setpriority)'
setpriority(PRIO_PROCESS, 0, -10) = 0

# trusty
On trusty with the 4.4 linux-generic-lts-xenial kernel, I found that '_newselect' is not a valid syscall, so snap run --strace fails:

$ sudo snap run --strace test-snapd-tools.cmd nice -n -10 uptime 2>&1
/usr/bin/strace: invalid system call '_newselect'
error: exit status 1

Using the old way without _newselect strace doesn't report it at all (just see the write() that nice failed but no syscall_* or sepriority).

Stracing nice outside of the snap command has strace finding setpriority:

$ sudo strace -e '!select,pselect6,clock_gettime' -f -D -vv nice -n -10 uptime 2>&1|grep setprio
setpriority(PRIO_PROCESS, 0, 4294967286) = 0

Please note in all Ubuntu releases the setpriority call was correctly mediated; it is just that on xenial and trusty the strace output was mysterious.

[Jamie Strandboge (jdstrand)]

Looking at the versions of strace, xenial has 4.11, trusty 4.8 and bionic 4.21. Upstream is very active: https://github.com/strace/strace/commits/v4.21 but while bionic has 4.21, if I remove strace from xenial and snap install strace-static --edge, I get 4.23 and similar behavior, so there is something more going on:

$ sudo snap run --strace test-snapd-tools.cmd nice -n -10 uptime 2>&1 | grep syscall
[pid 6358] syscall_0xffffffffffffffff(0, 0, 0xfffffff6, 0x865, 0, 0x1999999999999999) = -1 EPERM (Operation not permitted)

[Zygmunt Krynicki (zyga)]

I ran into this issue myself while working with the field and certification sprints during the Malta engineering sprint. I think we should either default to strace-from-snap or at least detect when strace is too old for the running kernel.

I'm marking it as triaged and low.