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# HG changeset patch
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# User Jed Davis <jld@mozilla.com>
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# Date 1526943705 21600
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# Node ID 6bb3adfa15c6877f7874429462dad88f8c978c4f
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# Parent 4c71c8454879c841871ecf3afb7dbdc96bad97fc
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Bug 1436242 - Avoid undefined behavior in IPC fd-passing code. r=froydnj
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MozReview-Commit-ID: 3szIPUssgF5
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diff --git a/ipc/chromium/src/chrome/common/ipc_channel_posix.cc b/ipc/chromium/src/chrome/common/ipc_channel_posix.cc
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--- a/ipc/chromium/src/chrome/common/ipc_channel_posix.cc
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+++ b/ipc/chromium/src/chrome/common/ipc_channel_posix.cc
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@@ -418,20 +418,37 @@ bool Channel::ChannelImpl::ProcessIncomi
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const int* fds;
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unsigned num_fds;
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unsigned fds_i = 0; // the index of the first unused descriptor
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if (input_overflow_fds_.empty()) {
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fds = wire_fds;
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num_fds = num_wire_fds;
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} else {
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- const size_t prev_size = input_overflow_fds_.size();
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- input_overflow_fds_.resize(prev_size + num_wire_fds);
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- memcpy(&input_overflow_fds_[prev_size], wire_fds,
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- num_wire_fds * sizeof(int));
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+ // This code may look like a no-op in the case where
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+ // num_wire_fds == 0, but in fact:
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+ //
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+ // 1. wire_fds will be nullptr, so passing it to memcpy is
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+ // undefined behavior according to the C standard, even though
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+ // the memcpy length is 0.
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+ //
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+ // 2. prev_size will be an out-of-bounds index for
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+ // input_overflow_fds_; this is undefined behavior according to
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+ // the C++ standard, even though the element only has its
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+ // pointer taken and isn't accessed (and the corresponding
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+ // operation on a C array would be defined).
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+ //
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+ // UBSan makes #1 a fatal error, and assertions in libstdc++ do
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+ // the same for #2 if enabled.
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+ if (num_wire_fds > 0) {
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+ const size_t prev_size = input_overflow_fds_.size();
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+ input_overflow_fds_.resize(prev_size + num_wire_fds);
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+ memcpy(&input_overflow_fds_[prev_size], wire_fds,
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+ num_wire_fds * sizeof(int));
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+ }
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fds = &input_overflow_fds_[0];
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num_fds = input_overflow_fds_.size();
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}
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// The data for the message we're currently reading consists of any data
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// stored in incoming_message_ followed by data in input_buf_ (followed by
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// other messages).
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