From d836d9e345865245bab28100a6065d6fa7b6a00c Mon Sep 17 00:00:00 2001 From: David Gibson Date: Wed, 18 Sep 2024 20:44:06 +1000 Subject: [PATCH] util: Remove possible quadratic behaviour from write_remainder() write_remainder() steps through the buffers in an IO vector writing out everything past a certain byte offset. However, on each iteration it rescans the buffer from the beginning to find out where we're up to. With an unfortunate set of write sizes this could lead to quadratic behaviour. In an even less likely set of circumstances (total vector length > maximum size_t) the 'skip' variable could overflow. This is one factor in a longstanding Coverity error we've seen (although I still can't figure out the remainder of its complaint). Rework write_remainder() to always work out our new position in the vector relative to our old/current position, rather than starting from the beginning each time. As a bonus this seems to fix the Coverity error. Signed-off-by: David Gibson Reviewed-by: Markus Armbruster Signed-off-by: Stefano Brivio --- util.c | 27 +++++++++++++++++---------- 1 file changed, 17 insertions(+), 10 deletions(-) diff --git a/util.c b/util.c index 7db7c2e..87309c5 100644 --- a/util.c +++ b/util.c @@ -597,10 +597,15 @@ int write_all_buf(int fd, const void *buf, size_t len) size_t left = len; while (left) { - ssize_t rc = write(fd, p, left); + ssize_t rc; + + do + rc = write(fd, p, left); + while ((rc < 0) && errno == EINTR); if (rc < 0) return -1; + p += rc; left -= rc; } @@ -615,28 +620,30 @@ int write_all_buf(int fd, const void *buf, size_t len) * * Return: 0 on success, -1 on error (with errno set) * - * #syscalls write writev + * #syscalls writev */ int write_remainder(int fd, const struct iovec *iov, size_t iovcnt, size_t skip) { - size_t offset, i; + size_t i = 0, offset; - while ((i = iov_skip_bytes(iov, iovcnt, skip, &offset)) < iovcnt) { + while ((i += iov_skip_bytes(iov + i, iovcnt - i, skip, &offset)) < iovcnt) { ssize_t rc; if (offset) { - rc = write(fd, (char *)iov[i].iov_base + offset, - iov[i].iov_len - offset); - } else { - rc = writev(fd, &iov[i], iovcnt - i); + /* Write the remainder of the partially written buffer */ + if (write_all_buf(fd, (char *)iov[i].iov_base + offset, + iov[i].iov_len - offset) < 0) + return -1; + i++; } + /* Write as much of the remaining whole buffers as we can */ + rc = writev(fd, &iov[i], iovcnt - i); if (rc < 0) return -1; - skip += rc; + skip = rc; } - return 0; }