glusterfsd crash caused by unprotected list traversal in gf_server_check_setxattr_cmd()

[jiankyu]

glusterfs version: v10.5

We encountered a glusterfsd crash issue recently, after analyzing the core dump, we are able to identify the crash happens in the server-helpers.c file, inside the gf_server_check_setxattr_cmd() function. When it traverses the xptr_list, it hit a nullptr, then when it wants to access the xprt->total_bytes_read, it hit the segmentation fault.

Access to the conf->xprt_list member must be protected by the conf->mutex lock, all other places follow this rule, unfortunately this function doesn't.

This problem exists in all active branches. (release-10, release-11, devel).