In addition, the trap isn't usable safely with shared libraries that because of signals. For example, glibc once it adopts rseq will block all signals during thread-creation making it impossible to use RET_TRAP. That's an issue that Firefox/Chromium has already run into and is one of the reasons why they are interested in switching to the seccomp notifier.
The trap also doesn't allow to continue syscalls nicely and - as Sargun pointed out - will require ptrace() to be used to inspect syscall arguments and so on. The notifier also has built-in protection against pid recycling, is more secure and is way more efficient.
This is somewhat unrelated to the seccomp notifier but since ptrace() came up I want to lose a few words about it. (And this is more a criticism of the interface not the implementation. I love Oleg who maintains it and is one of the few people who understand all its intricacies!)
As a rule of thumb: you can do almost anything with ptrace(). Which is why people not really putting an effort into kernel patch reviews often come up with the argument "Why do you need a separate api for that. You can already do that with ptrace().". To which the correct answer in my book almost always is: "Because it would be a horrible hack." Effectively, when introducing a dedicated api to do something that you can in some shape or form do with ptrace() is the equivalent to moving it from a debugging hack to a (hopefully well-designed) feature.
Hell, the history of CRIU is essentially the history of building apis out of ptrace() hacks (I'm being facetious of course.).
Imho, with ptrace() you're always in non-cooperative mode to some extent, i.e. you force the behavior on the task. The whole kernel code for ptrace() attach is literally "I'm your parent now." whereas features such as the notifier are almost always cooperative since the task itself is doing the work. Specifically for the notifier the nice thing is that all the work is happening in the task itself. This is especially relevant when you e.g. install file descriptors into the task which is a future patchset that is about to be merged.
This is somewhat unrelated to the seccomp notifier but since ptrace() came up I want to lose a few words about it. (And this is more a criticism of the interface not the implementation. I love Oleg who maintains it and is one of the few people who understand all its intricacies!)
As a rule of thumb: you can do almost anything with ptrace(). Which is why people not really putting an effort into kernel patch reviews often come up with the argument "Why do you need a separate api for that. You can already do that with ptrace().". To which the correct answer in my book almost always is: "Because it would be a horrible hack." Effectively, when introducing a dedicated api to do something that you can in some shape or form do with ptrace() is the equivalent to moving it from a debugging hack to a (hopefully well-designed) feature.
Hell, the history of CRIU is essentially the history of building apis out of ptrace() hacks (I'm being facetious of course.).
Imho, with ptrace() you're always in non-cooperative mode to some extent, i.e. you force the behavior on the task. The whole kernel code for ptrace() attach is literally "I'm your parent now." whereas features such as the notifier are almost always cooperative since the task itself is doing the work. Specifically for the notifier the nice thing is that all the work is happening in the task itself. This is especially relevant when you e.g. install file descriptors into the task which is a future patchset that is about to be merged.