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always try the time64 syscall first since we can use its success to
conclude that no conversion is needed (any setsockopt for the
timestamp options would have succeeded without need for fallbacks).
otherwise, we have to remember the original controllen for each
msghdr, requiring O(vlen) space, so vlen must be bounded. linux clamps
it to IOV_MAX for sendmmsg only (not recvmmsg), but doing the same for
recvmmsg is not unreasonable, especially since the limitation will
only apply to old kernels.
we could optimize to avoid trying SYS_recvmmsg_time64 first if all
msghdrs have controllen zero, or support unlimited vlen by looping and
emulating the timeout logic, but I'm not inclined to do complex and
error-prone optimizations on a function that has so many underlying
problems it should really never be used.
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somewhat analogous to commit d0b547dfb5f7678cab6bc39dd736ed6454357ca4,
but here the omission of the null timeout check was in the time64
syscall code path. this code is not yet used except on x32.
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the time64 syscall is used only if the timeout does not fit in 32
bits. after preprocessing, the code is unchanged on 64-bit archs. for
32-bit archs, the timeout now goes through an intermediate copy,
meaning that the caller does not get back the updated timeout. this is
based on my reading of the documentation, which does not document the
updating as a contract you can rely on, and mentions that the whole
recvmmsg timeout mechanism is buggy and unlikely to be useful. if it
turns out that there's interest in making the remaining time
officially available to callers, such functionality could be added
back later.
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these are not pure syscall wrappers because they have to work around
kernel API bugs on 64-bit archs. the workarounds could probably be
made somewhat more efficient, but at the cost of more complexity. this
may be revisited later.
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