We received an answer from IBM.
So be carrefull if you are using informix on AIX 7.2 and want to install TL5.
there was change in 7.2 TL5 (all current SP's) in the behavior of the setreuid() function to adhere to Unix10 standards. Basically, it limited the ability to switch from root to an non-root UID and then back to root as per the standards (and as per our manpage):
Under setreuid: If both the real user ID and effective user ID are changed, the saved user ID is set to the new effective user ID. Note that this change results in a loss of original privileges.
Unfortunately this change was a bit to aggressive in that it also prevented one from switching back when a UID of -1 was specified and prevented setreuid() from switching back to root when it should have been allowed (e.g. when one or the other [ruid or euid] should have still allowed this operation. The ifixes corrects this overly restrictive behavior.It currently has not yet been assigned an APAR number, but we are expecting this to occur shortly. For now we would like you to try the ifix with Informix to see if it resolves the issue. We are expecting it will as once you are able to switch back to euid=root setreuid(), the functions that are failing later with EPERM should be allowed.Please let us know if you have any additional questions or concerns with this
We haven't tested that specifically, but running as user informix is security best practice anyway.