this is the call sequence from the direct call of mysqladmin:

[...]
stat("/home/super/.my.cnf", {st_mode=S_IFREG|0600, st_size=118, ...}) = 0
open("/home/super/.my.cnf", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0600, st_size=118, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f4a65649000
read(3, "[xtrabackup]\nuser=root\npassword="..., 4096) = 118
read(3, "", 4096) = 0
close(3)
[...]

and this is the same sequence from the mysqladmin instance called by logrotate:

[...]
stat("/home/super/.my.cnf", {st_mode=S_IFREG|0600, st_size=0, ...}) = 0
open("/home/super/.my.cnf", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0600, st_size=0, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f18fe4f4000
read(3, "", 4096) = 0
close(3) = 0
[...]