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git-commit-vandalism/send-pack.c
Jeff King 739cf49161 send-pack: close demux pipe before finishing async process 
This fixes a deadlock on the client side when pushing a
large number of refs from a corrupted repo.  There's a
reproduction script below, but let's start with a
human-readable explanation.

The client side of a push goes something like this:

  1. Start an async process to demux sideband coming from
     the server.

  2. Run pack-objects to send the actual pack, and wait for
     its status via finish_command().

  3. If pack-objects failed, abort immediately.

  4. If pack-objects succeeded, read the per-ref status from
     the server, which is actually coming over a pipe from
     the demux process started in step 1.

We run finish_async() to wait for and clean up the demux
process in two places. In step 3, if we see an error, we
want it to end early. And after step 4, it should be done
writing any data and we are just cleaning it up.

Let's focus on the error case first. We hand the output
descriptor to the server over to pack-objects. So by the
time it has returned an error to us, it has closed the
descriptor and the server has gotten EOF. The server will
mark all refs as failed with "unpacker error" and send us
back the status for each (followed by EOF).

This status goes to the demuxer thread, which relays it over
a pipe to the main thread. But the main thread never even
tries reading the status. It's trying to bail because of the
pack-objects error, and is waiting for the demuxer thread to
finish. If there are a small number of refs, that's OK; the
demuxer thread writes into the pipe buffer, sees EOF from
the server, and quits. But if there are a large number of
refs, it may block on write() back to the main thread,
leading to a deadlock (the main thread is waiting for the
demuxer to finish, the demuxer is waiting for the main
thread to read).

We can break this deadlock by closing the pipe between the
demuxer and the main thread before calling finish_async().
Then the demuxer gets a write() error and exits.

The non-error case usually just works, because we will have
read all of the data from the other side. We do close
demux.out already, but we only do so _after_ calling
finish_async(). This is OK because there shouldn't be any
more data coming from the server. But technically we've only
read to a flush packet, and a broken or malicious server
could be sending more cruft. In such a case, we would hit
the same deadlock. Closing the pipe first doesn't affect the
normal case, and means that for a cruft-sending server,
we'll notice a write() error rather than deadlocking.

Note that when write() sees this error, we'll actually
deliver SIGPIPE to the thread, which will take down the
whole process (unless we're compiled with NO_PTHREADS). This
isn't ideal, but it's an improvement over the status quo,
which is deadlocking. And SIGPIPE handling in async threads
is a bigger problem that we can deal with separately.

A simple reproduction for the error case is below. It's
technically racy (we could exit the main process and take
down the async thread with us before it even reads the
status), though in practice it seems to fail pretty
consistently.

    git init repo &&
    cd repo &&

    # make some commits; we need two so we can simulate corruption
    # in the history later.
    git commit --allow-empty -m one &&
    one=$(git rev-parse HEAD) &&
    git commit --allow-empty -m two &&
    two=$(git rev-parse HEAD) &&

    # now make a ton of refs; our goal here is to overflow the pipe buffer
    # when reporting the ref status, which will cause the demuxer to block
    # on write()
    for i in $(seq 20000); do
    	echo "create refs/heads/this-is-a-really-long-branch-name-$i $two"
    done |
    git update-ref --stdin &&

    # now make a corruption in the history such that pack-objects will fail
    rm -vf .git/objects/$(echo $one | sed 's}..}&/}') &&

    # and then push the result
    git init --bare dst.git &&
    git push --mirror dst.git

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2016-04-20 13:33:53 -07:00

581 lines
14 KiB
C
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#include "builtin.h"
#include "commit.h"
#include "refs.h"
#include "pkt-line.h"
#include "sideband.h"
#include "run-command.h"
#include "remote.h"
#include "connect.h"
#include "send-pack.h"
#include "quote.h"
#include "transport.h"
#include "version.h"
#include "sha1-array.h"
#include "gpg-interface.h"
#include "cache.h"
int option_parse_push_signed(const struct option *opt,
const char *arg, int unset)
{
if (unset) {
*(int *)(opt->value) = SEND_PACK_PUSH_CERT_NEVER;
return 0;
}
switch (git_parse_maybe_bool(arg)) {
case 1:
*(int *)(opt->value) = SEND_PACK_PUSH_CERT_ALWAYS;
return 0;
case 0:
*(int *)(opt->value) = SEND_PACK_PUSH_CERT_NEVER;
return 0;
}
if (!strcasecmp("if-asked", arg)) {
*(int *)(opt->value) = SEND_PACK_PUSH_CERT_IF_ASKED;
return 0;
}
die("bad %s argument: %s", opt->long_name, arg);
}
static int feed_object(const unsigned char *sha1, int fd, int negative)
{
char buf[42];
if (negative && !has_sha1_file(sha1))
return 1;
memcpy(buf + negative, sha1_to_hex(sha1), 40);
if (negative)
buf[0] = '^';
buf[40 + negative] = '\n';
return write_or_whine(fd, buf, 41 + negative, "send-pack: send refs");
}
/*
* Make a pack stream and spit it out into file descriptor fd
*/
static int pack_objects(int fd, struct ref *refs, struct sha1_array *extra, struct send_pack_args *args)
{
/*
* The child becomes pack-objects --revs; we feed
* the revision parameters to it via its stdin and
* let its stdout go back to the other end.
*/
const char *argv[] = {
"pack-objects",
"--all-progress-implied",
"--revs",
"--stdout",
NULL,
NULL,
NULL,
NULL,
NULL,
NULL,
};
struct child_process po = CHILD_PROCESS_INIT;
int i;
i = 4;
if (args->use_thin_pack)
argv[i++] = "--thin";
if (args->use_ofs_delta)
argv[i++] = "--delta-base-offset";
if (args->quiet || !args->progress)
argv[i++] = "-q";
if (args->progress)
argv[i++] = "--progress";
if (is_repository_shallow())
argv[i++] = "--shallow";
po.argv = argv;
po.in = -1;
po.out = args->stateless_rpc ? -1 : fd;
po.git_cmd = 1;
if (start_command(&po))
die_errno("git pack-objects failed");
/*
* We feed the pack-objects we just spawned with revision
* parameters by writing to the pipe.
*/
for (i = 0; i < extra->nr; i++)
if (!feed_object(extra->sha1[i], po.in, 1))
break;
while (refs) {
if (!is_null_oid(&refs->old_oid) &&
!feed_object(refs->old_oid.hash, po.in, 1))
break;
if (!is_null_oid(&refs->new_oid) &&
!feed_object(refs->new_oid.hash, po.in, 0))
break;
refs = refs->next;
}
close(po.in);
if (args->stateless_rpc) {
char *buf = xmalloc(LARGE_PACKET_MAX);
while (1) {
ssize_t n = xread(po.out, buf, LARGE_PACKET_MAX);
if (n <= 0)
break;
send_sideband(fd, -1, buf, n, LARGE_PACKET_MAX);
}
free(buf);
close(po.out);
po.out = -1;
}
if (finish_command(&po))
return -1;
return 0;
}
static int receive_status(int in, struct ref *refs)
{
struct ref *hint;
int ret = 0;
char *line = packet_read_line(in, NULL);
if (!starts_with(line, "unpack "))
return error("did not receive remote status");
if (strcmp(line, "unpack ok")) {
error("unpack failed: %s", line + 7);
ret = -1;
}
hint = NULL;
while (1) {
char *refname;
char *msg;
line = packet_read_line(in, NULL);
if (!line)
break;
if (!starts_with(line, "ok ") && !starts_with(line, "ng ")) {
error("invalid ref status from remote: %s", line);
ret = -1;
break;
}
refname = line + 3;
msg = strchr(refname, ' ');
if (msg)
*msg++ = '\0';
/* first try searching at our hint, falling back to all refs */
if (hint)
hint = find_ref_by_name(hint, refname);
if (!hint)
hint = find_ref_by_name(refs, refname);
if (!hint) {
warning("remote reported status on unknown ref: %s",
refname);
continue;
}
if (hint->status != REF_STATUS_EXPECTING_REPORT) {
warning("remote reported status on unexpected ref: %s",
refname);
continue;
}
if (line[0] == 'o' && line[1] == 'k')
hint->status = REF_STATUS_OK;
else {
hint->status = REF_STATUS_REMOTE_REJECT;
ret = -1;
}
if (msg)
hint->remote_status = xstrdup(msg);
/* start our next search from the next ref */
hint = hint->next;
}
return ret;
}
static int sideband_demux(int in, int out, void *data)
{
int *fd = data, ret;
#ifdef NO_PTHREADS
close(fd[1]);
#endif
ret = recv_sideband("send-pack", fd[0], out);
close(out);
return ret;
}
static int advertise_shallow_grafts_cb(const struct commit_graft *graft, void *cb)
{
struct strbuf *sb = cb;
if (graft->nr_parent == -1)
packet_buf_write(sb, "shallow %s\n", oid_to_hex(&graft->oid));
return 0;
}
static void advertise_shallow_grafts_buf(struct strbuf *sb)
{
if (!is_repository_shallow())
return;
for_each_commit_graft(advertise_shallow_grafts_cb, sb);
}
#define CHECK_REF_NO_PUSH -1
#define CHECK_REF_STATUS_REJECTED -2
#define CHECK_REF_UPTODATE -3
static int check_to_send_update(const struct ref *ref, const struct send_pack_args *args)
{
if (!ref->peer_ref && !args->send_mirror)
return CHECK_REF_NO_PUSH;
/* Check for statuses set by set_ref_status_for_push() */
switch (ref->status) {
case REF_STATUS_REJECT_NONFASTFORWARD:
case REF_STATUS_REJECT_ALREADY_EXISTS:
case REF_STATUS_REJECT_FETCH_FIRST:
case REF_STATUS_REJECT_NEEDS_FORCE:
case REF_STATUS_REJECT_STALE:
case REF_STATUS_REJECT_NODELETE:
return CHECK_REF_STATUS_REJECTED;
case REF_STATUS_UPTODATE:
return CHECK_REF_UPTODATE;
default:
return 0;
}
}
/*
* the beginning of the next line, or the end of buffer.
*
* NEEDSWORK: perhaps move this to git-compat-util.h or somewhere and
* convert many similar uses found by "git grep -A4 memchr".
*/
static const char *next_line(const char *line, size_t len)
{
const char *nl = memchr(line, '\n', len);
if (!nl)
return line + len; /* incomplete line */
return nl + 1;
}
static int generate_push_cert(struct strbuf *req_buf,
const struct ref *remote_refs,
struct send_pack_args *args,
const char *cap_string,
const char *push_cert_nonce)
{
const struct ref *ref;
char *signing_key = xstrdup(get_signing_key());
const char *cp, *np;
struct strbuf cert = STRBUF_INIT;
int update_seen = 0;
strbuf_addf(&cert, "certificate version 0.1\n");
strbuf_addf(&cert, "pusher %s ", signing_key);
datestamp(&cert);
strbuf_addch(&cert, '\n');
if (args->url && *args->url) {
char *anon_url = transport_anonymize_url(args->url);
strbuf_addf(&cert, "pushee %s\n", anon_url);
free(anon_url);
}
if (push_cert_nonce[0])
strbuf_addf(&cert, "nonce %s\n", push_cert_nonce);
strbuf_addstr(&cert, "\n");
for (ref = remote_refs; ref; ref = ref->next) {
if (check_to_send_update(ref, args) < 0)
continue;
update_seen = 1;
strbuf_addf(&cert, "%s %s %s\n",
oid_to_hex(&ref->old_oid),
oid_to_hex(&ref->new_oid),
ref->name);
}
if (!update_seen)
goto free_return;
if (sign_buffer(&cert, &cert, signing_key))
die(_("failed to sign the push certificate"));
packet_buf_write(req_buf, "push-cert%c%s", 0, cap_string);
for (cp = cert.buf; cp < cert.buf + cert.len; cp = np) {
np = next_line(cp, cert.buf + cert.len - cp);
packet_buf_write(req_buf,
"%.*s", (int)(np - cp), cp);
}
packet_buf_write(req_buf, "push-cert-end\n");
free_return:
free(signing_key);
strbuf_release(&cert);
return update_seen;
}
static int atomic_push_failure(struct send_pack_args *args,
struct ref *remote_refs,
struct ref *failing_ref)
{
struct ref *ref;
/* Mark other refs as failed */
for (ref = remote_refs; ref; ref = ref->next) {
if (!ref->peer_ref && !args->send_mirror)
continue;
switch (ref->status) {
case REF_STATUS_EXPECTING_REPORT:
ref->status = REF_STATUS_ATOMIC_PUSH_FAILED;
continue;
default:
break; /* do nothing */
}
}
return error("atomic push failed for ref %s. status: %d\n",
failing_ref->name, failing_ref->status);
}
#define NONCE_LEN_LIMIT 256
static void reject_invalid_nonce(const char *nonce, int len)
{
int i = 0;
if (NONCE_LEN_LIMIT <= len)
die("the receiving end asked to sign an invalid nonce <%.*s>",
len, nonce);
for (i = 0; i < len; i++) {
int ch = nonce[i] & 0xFF;
if (isalnum(ch) ||
ch == '-' || ch == '.' ||
ch == '/' || ch == '+' ||
ch == '=' || ch == '_')
continue;
die("the receiving end asked to sign an invalid nonce <%.*s>",
len, nonce);
}
}
int send_pack(struct send_pack_args *args,
int fd[], struct child_process *conn,
struct ref *remote_refs,
struct sha1_array *extra_have)
{
int in = fd[0];
int out = fd[1];
struct strbuf req_buf = STRBUF_INIT;
struct strbuf cap_buf = STRBUF_INIT;
struct ref *ref;
int need_pack_data = 0;
int allow_deleting_refs = 0;
int status_report = 0;
int use_sideband = 0;
int quiet_supported = 0;
int agent_supported = 0;
int use_atomic = 0;
int atomic_supported = 0;
unsigned cmds_sent = 0;
int ret;
struct async demux;
const char *push_cert_nonce = NULL;
/* Does the other end support the reporting? */
if (server_supports("report-status"))
status_report = 1;
if (server_supports("delete-refs"))
allow_deleting_refs = 1;
if (server_supports("ofs-delta"))
args->use_ofs_delta = 1;
if (server_supports("side-band-64k"))
use_sideband = 1;
if (server_supports("quiet"))
quiet_supported = 1;
if (server_supports("agent"))
agent_supported = 1;
if (server_supports("no-thin"))
args->use_thin_pack = 0;
if (server_supports("atomic"))
atomic_supported = 1;
if (args->push_cert != SEND_PACK_PUSH_CERT_NEVER) {
int len;
push_cert_nonce = server_feature_value("push-cert", &len);
if (push_cert_nonce) {
reject_invalid_nonce(push_cert_nonce, len);
push_cert_nonce = xmemdupz(push_cert_nonce, len);
} else if (args->push_cert == SEND_PACK_PUSH_CERT_ALWAYS) {
die(_("the receiving end does not support --signed push"));
} else if (args->push_cert == SEND_PACK_PUSH_CERT_IF_ASKED) {
warning(_("not sending a push certificate since the"
" receiving end does not support --signed"
" push"));
}
}
if (!remote_refs) {
fprintf(stderr, "No refs in common and none specified; doing nothing.\n"
"Perhaps you should specify a branch such as 'master'.\n");
return 0;
}
if (args->atomic && !atomic_supported)
die(_("the receiving end does not support --atomic push"));
use_atomic = atomic_supported && args->atomic;
if (status_report)
strbuf_addstr(&cap_buf, " report-status");
if (use_sideband)
strbuf_addstr(&cap_buf, " side-band-64k");
if (quiet_supported && (args->quiet || !args->progress))
strbuf_addstr(&cap_buf, " quiet");
if (use_atomic)
strbuf_addstr(&cap_buf, " atomic");
if (agent_supported)
strbuf_addf(&cap_buf, " agent=%s", git_user_agent_sanitized());
/*
* NEEDSWORK: why does delete-refs have to be so specific to
* send-pack machinery that set_ref_status_for_push() cannot
* set this bit for us???
*/
for (ref = remote_refs; ref; ref = ref->next)
if (ref->deletion && !allow_deleting_refs)
ref->status = REF_STATUS_REJECT_NODELETE;
if (!args->dry_run)
advertise_shallow_grafts_buf(&req_buf);
if (!args->dry_run && push_cert_nonce)
cmds_sent = generate_push_cert(&req_buf, remote_refs, args,
cap_buf.buf, push_cert_nonce);
/*
* Clear the status for each ref and see if we need to send
* the pack data.
*/
for (ref = remote_refs; ref; ref = ref->next) {
switch (check_to_send_update(ref, args)) {
case 0: /* no error */
break;
case CHECK_REF_STATUS_REJECTED:
/*
* When we know the server would reject a ref update if
* we were to send it and we're trying to send the refs
* atomically, abort the whole operation.
*/
if (use_atomic)
return atomic_push_failure(args, remote_refs, ref);
/* Fallthrough for non atomic case. */
default:
continue;
}
if (!ref->deletion)
need_pack_data = 1;
if (args->dry_run || !status_report)
ref->status = REF_STATUS_OK;
else
ref->status = REF_STATUS_EXPECTING_REPORT;
}
/*
* Finally, tell the other end!
*/
for (ref = remote_refs; ref; ref = ref->next) {
char *old_hex, *new_hex;
if (args->dry_run || push_cert_nonce)
continue;
if (check_to_send_update(ref, args) < 0)
continue;
old_hex = oid_to_hex(&ref->old_oid);
new_hex = oid_to_hex(&ref->new_oid);
if (!cmds_sent) {
packet_buf_write(&req_buf,
"%s %s %s%c%s",
old_hex, new_hex, ref->name, 0,
cap_buf.buf);
cmds_sent = 1;
} else {
packet_buf_write(&req_buf, "%s %s %s",
old_hex, new_hex, ref->name);
}
}
if (args->stateless_rpc) {
if (!args->dry_run && (cmds_sent || is_repository_shallow())) {
packet_buf_flush(&req_buf);
send_sideband(out, -1, req_buf.buf, req_buf.len, LARGE_PACKET_MAX);
}
} else {
write_or_die(out, req_buf.buf, req_buf.len);
packet_flush(out);
}
strbuf_release(&req_buf);
strbuf_release(&cap_buf);
if (use_sideband && cmds_sent) {
memset(&demux, 0, sizeof(demux));
demux.proc = sideband_demux;
demux.data = fd;
demux.out = -1;
if (start_async(&demux))
die("send-pack: unable to fork off sideband demultiplexer");
in = demux.out;
}
if (need_pack_data && cmds_sent) {
if (pack_objects(out, remote_refs, extra_have, args) < 0) {
for (ref = remote_refs; ref; ref = ref->next)
ref->status = REF_STATUS_NONE;
if (args->stateless_rpc)
close(out);
if (git_connection_is_socket(conn))
shutdown(fd[0], SHUT_WR);
if (use_sideband) {
close(demux.out);
finish_async(&demux);
}
fd[1] = -1;
return -1;
}
if (!args->stateless_rpc)
/* Closed by pack_objects() via start_command() */
fd[1] = -1;
}
if (args->stateless_rpc && cmds_sent)
packet_flush(out);
if (status_report && cmds_sent)
ret = receive_status(in, remote_refs);
else
ret = 0;
if (args->stateless_rpc)
packet_flush(out);
if (use_sideband && cmds_sent) {
close(demux.out);
if (finish_async(&demux)) {
error("error in sideband demultiplexer");
ret = -1;
}
}
if (ret < 0)
return ret;
if (args->porcelain)
return 0;
for (ref = remote_refs; ref; ref = ref->next) {
switch (ref->status) {
case REF_STATUS_NONE:
case REF_STATUS_UPTODATE:
case REF_STATUS_OK:
break;
default:
return -1;
}
}
return 0;
}
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