[xen staging] tools/libxenstat: Fix infinite loop when QEMU dies

[patchbot@xxxxxxx]

commit cd5058316c2ce4966fd7c2852216c47a0dc11618
Author:     Hans Reiser <hr@xxxxxxxxxxxxxxxxx>
AuthorDate: Mon Nov 9 14:36:00 2020 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Jan 22 14:28:44 2021 +0000

    tools/libxenstat: Fix infinite loop when QEMU dies
    
    Occasionally, "dead" xentop processes consuming 100% CPU time have been
    observed.
    
    When the QEMU process the qmp_read() function is communicating with
    terminates, qmp_read() may enter an infinite loop.  poll() signals EOF 
(POLLIN
    and POLLHUP set), the subsequent read() call returns 0, and then the 
function
    calls poll() again, which still sees the EOF condition and will return again
    immediately with POLLIN and POLLHUP set, repeating ad infinitum.
    
    A simple fix is to terminate the loop when read returns 0 (under "normal"
    instances, poll will return with POLLIN set only if there is data to read, 
so
    read will always read >0 bytes, except if the socket has been closed).
    
    Signed-off-by: Hans Reiser <hr@xxxxxxxxxxxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 tools/libs/stat/xenstat_qmp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/libs/stat/xenstat_qmp.c b/tools/libs/stat/xenstat_qmp.c
index 19b236e7b6..0c5748ba68 100644
--- a/tools/libs/stat/xenstat_qmp.c
+++ b/tools/libs/stat/xenstat_qmp.c
@@ -298,7 +298,7 @@ static int qmp_read(int qfd, unsigned char **qstats)
        pfd[0].events = POLLIN;
        while ((n = poll(pfd, 1, 10)) > 0) {
                if (pfd[0].revents & POLLIN) {
-                       if ((n = read(qfd, buf, sizeof(buf))) < 0) {
+                       if ((n = read(qfd, buf, sizeof(buf))) <= 0) {
                                free(*qstats);
                                return 0;
                        }
--
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