mysqld exits immediately with nothing in log, file read failures in strace

I want to run mysqld --skip-grant-tables (to reset the root password).

$ sudo mysqld --skip-grant-tables --explicit_defaults_for_timestamp=ON
2016-11-17 11:27:16 0 [Note] mysqld (mysqld 5.6.31-0ubuntu0.15.10.1) starting as process 5432 ...

Then it exits immediately. Nothing is produced in /var/log/mysql. mysqld is not already running, and until I killed it a minute ago, it was working fine.

EDIT I pulled these suspicious lines out of strace

open("/var/log/mysql/error.log", O_WRONLY|O_CREAT|O_APPEND, 0666) = -1 EACCES (Permission denied)
^---- (error.log doesn't exist and I'm root so wtf?)
...
readlink("./mysql/plugin.MYI", 0x7ffe24c368c0, 511) = -1 EINVAL (Invalid argument)
readlink("./mysql/plugin.MYD", 0x7ffe24c36ac0, 511) = -1 EINVAL (Invalid argument)
access("./mysql/plugin.TRG", F_OK)      = -1 ENOENT (No such file or directory)
open("./ibdata1", O_RDWR|O_CREAT|O_EXCL, 0660) = -1 EEXIST (File exists)
stat("./ib_logfile2", 0x7ffe24c32e70)   = -1 ENOENT (No such file or directory)
stat("./undo001", 0x7ffe24c31e30)       = -1 ENOENT (No such file or directory)
open("./undo001", O_RDWR)               = -1 ENOENT (No such file or directory)
open("./mysql/innodb_index_stats.isl", O_RDWR) = -1 ENOENT (No such file or directory)
...
open("./phpmyadmin/pma__users.isl", O_RDWR) = -1 ENOENT (No such file or directory)
...
stat("/root/.mylogin.cnf", 0x7ffe24c36e50) = -1 EACCES (Permission denied)
unlink("/var/run/mysqld/mysqld.sock")   = -1 ENOENT (No such file or directory)
bind(12, {sa_family=AF_LOCAL, sun_path="/var/run/mysqld/mysqld.sock"}, 110) = -1 ENOENT (No such file or directory)

What happened?

su - mysql -c 'mysqld --skip-grant-tables --explicit_defaults_for_timestamp=ON'