musl - Calling setxid() in a vfork()-child

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Message-ID: <93cbaeffbc860a145843e0380058c50e@ispras.ru>
Date: Mon, 12 Oct 2020 12:27:44 +0300
From: Alexey Izbyshev <izbyshev@...ras.ru>
To: musl@...ts.openwall.com
Subject: Calling setxid() in a vfork()-child

Hello,

I'm investigating possibility of using vfork() instead of fork() in a 
Linux-only application. Before calling execve(), the app might need to 
call some functions to setup the child, including setxid() (let's assume 
that security concerns of [1] are not applicable). I'm aware that POSIX 
doesn't allow that for vfork()-children, but I'm also aware that it 
might be OK on Linux if the set of functions is sufficiently 
constrained, and that vfork() is used to efficiently implement 
posix_spawn() in C libraries. However, setuid()/setgid() seem 
particularly tricky because of the need to call the actual syscall in 
all threads, so if a C library is unaware that setxid() is called in a 
vfork()-child, it might attempt to interact with threads of the parent 
process, potentially causing trouble. I've checked musl and found a 
recent commit[2] that fixes this exact issue. I've also checked 
glibc[3], but haven't found any handling of this case (and vfork() 
doesn't appear to do anything special in this regard either[4]).

Do I understand correctly that, from an application developer 
perspective, it's currently better to avoid setxid/setrlimit libc 
functions in a vfork()-child, and that using syscall() or avoiding 
vfork() entirely is preferred in this case?

Thanks,
Alexey

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=14749
[2] https://git.musl-libc.org/cgit/musl/commit/?id=a5aff1972
[3] 
https://sourceware.org/git?p=glibc.git;a=blob;f=nptl/allocatestack.c;h=4b45f8c884b;hb=HEAD#l1082
[4] 
https://sourceware.org/git?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/x86_64/vfork.S;h=776d2fc61;hb=HEAD#l44

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