DROPBEAR_SVR_MULTIUSER sanity check false positive fail

[davidatrsp]

Hejsa
Disabling DROPBEAR_SVR_MULTIUSER config on kernel 5.10 without CONFIG_MULTIUSER produces a failed sanity check when initiation a session.
Traced the issue down to

dropbear/common-session.c

Line 75 in 9defeb4

getuid();

. The "getuid()" doesn't set errno to "ENOSYS" as expected.
Looked into the kernel source and looks like "getuid()" syscall is always present, independently of CONFIG_MULTIUSER. Also, when CONFIG_MULTIUSER is disabled "getuid()" always succeeds and returns "0". This behaviour is present at leats from kernel 4.9.
My workaround is to change "getuid" syscall into "getgroups". The "getgroups" has the expected behaviour, it is only present when the kernel is configured with CONFIG_MULTIUSER.

[mkj]

Thanks, I'll fix that. I wonder if that means no one has used the disabled-DROPBEAR_SVR_MULTIUSER functionality since it was added...