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From 591b3b168d949c45d5b5994332a3007767845434 Mon Sep 17 00:00:00 2001
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Message-Id: <591b3b168d949c45d5b5994332a3007767845434.1638277575.git.dcaratti@redhat.com>
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From: Paolo Abeni <paolo.abeni@gmail.com>
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Date: Thu, 14 Oct 2021 05:05:54 +0200
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Subject: [PATCH] mptcpize: use explicit file copy instead of rename() (#161)
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The mentioned syscall fails if the involved files belong to
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different fs, which is pretty much expected in the relevant
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scenario (tmp file, in tmpfs, and unit file usually under the
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root partition)
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Instead use sendfile() to explicitly copy all the contents. Note
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that we need to close and re-open the unit file, as sendfile()
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expect a O_WRITE fd as the target.
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---
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src/mptcpize.c | 24 ++++++++++++++++++++----
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1 file changed, 20 insertions(+), 4 deletions(-)
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diff --git a/src/mptcpize.c b/src/mptcpize.c
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index cb79e09..b502d75 100644
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--- a/src/mptcpize.c
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+++ b/src/mptcpize.c
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@@ -13,6 +13,7 @@
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#include <sys/types.h>
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#include <sys/stat.h>
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+#include <sys/sendfile.h>
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#include <argp.h>
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#include <dlfcn.h>
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@@ -163,10 +164,12 @@ static int unit_update(int argc, char *argv[], int enable)
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char *unit, *line = NULL;
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int append_env = enable;
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char dst_path[PATH_MAX];
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+ off_t bytes_copied = 0;
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+ struct stat fileinfo;
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+ int dst, unit_fd;
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size_t len = 0;
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ssize_t read;
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FILE *src;
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- int dst;
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if (argc < 1) {
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fprintf(stderr, "missing unit argument\n");
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@@ -210,11 +213,24 @@ static int unit_update(int argc, char *argv[], int enable)
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error(1, errno, "can't read from %s", unit);
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free(line);
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fclose(src);
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- close(dst);
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- if (rename(dst_path, unit) < 0)
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- error(1, errno, "can't rename %s to %s", dst_path, unit);
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+ // copy back the modified file into the original unit
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+ // note: avoid using rename, as it fails across filesystems
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+ if (fstat(dst, &fileinfo) < 0)
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+ error(1, errno, "can't stat %s", dst_path);
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+
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+ // re-open the unit file for writing
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+ // mkstemp already opened the temporary file for R/W so we don't need
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+ // to touch that file descriptor.
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+ unit_fd = open(unit, O_TRUNC | O_RDWR);
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+ if (unit_fd < 0)
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+ error(1, errno, "can't open %s for writing", unit);
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+ while (bytes_copied < fileinfo.st_size)
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+ if (sendfile(unit_fd, dst, &bytes_copied, fileinfo.st_size - bytes_copied) < 0)
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+ error(1, errno, "can't copy from %s to %s", dst_path, unit);
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+
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+ close(dst);
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if (system("systemctl daemon-reload") != 0)
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error(1, errno, "can't reload unit, manual 'systemctl daemon-reload' is required");
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--
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2.31.1
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