[Devel] [PATCH RHEL7 COMMIT] modules: use kvmalloc when creating sysfs attributes for ELF sections

[Vasily Averin]

The commit is pushed to "branch-rh7-3.10.0-1127.18.2.vz7.163.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1127.18.2.vz7.163.24
------>
commit 29cd62575fb2685a00ea2ff4f350a2ef11716c11
Author: Evgenii Shatokhin <eshatokhin at virtuozzo.com>
Date:   Fri Sep 18 12:52:06 2020 +0300

    modules: use kvmalloc when creating sysfs attributes for ELF sections
    
    A kernel module containing a ReadyKernel patch could have lots of ELF
    sections: one per each new or patched function, one per each new static
    or global variable, etc.
    
    The kernel creates a sysfs file /sys/module/<module_name>/sections/<section_name>
    for each loaded section when the patch module is being loaded, see
    add_sect_attrs() in kernel/module.c. A big chunk of memory is allocated
    for all these with kzalloc.
    
    For the ReadyKernel patches we have already released, the amount of memory
    could be as high as 34528 bytes (48 + 80 * 431 loaded sections), which is
    a 4th order allocation. 3rd order allocations are also common here,
    see https://jira.sw.ru/browse/PSBM-95050.
    
    Not only it is a waste (contiguous memory is not needed there), but the
    allocation may also fail when the memory is fragmented. ReadyKernel patches
    are often used in the systems with rather significant uptime, so the
    fragmentation is possible.
    
    It could be better if the patch modules did not use too many ELF sections.
    However, the KPatch maintainers pointed out (https://github.com/dynup/kpatch/pull/1131)
    that the same problem would affect regular kernel modules as well after
    FGKASLR has been merged into the mainline kernel. Combining the sections
    of the kernel modules destroys the purpose of FGKASLR, so, it was agreed
    that we probably should just switch to kvmalloc+kvfree in add_sect_attrs/free_sect_attrs.
    
    Details and discussion: https://www.spinics.net/lists/live-patching/msg06364.html
    
    https://jira.sw.ru/browse/PSBM-108015
    Signed-off-by: Evgenii Shatokhin <eshatokhin at virtuozzo.com>
---
 kernel/module.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index 799b365..8d54b57 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -1502,7 +1502,7 @@ static void free_sect_attrs(struct module_sect_attrs *sect_attrs)
 
 	for (section = 0; section < sect_attrs->nsections; section++)
 		kfree(sect_attrs->attrs[section].name);
-	kfree(sect_attrs);
+	kvfree(sect_attrs);
 }
 
 static void add_sect_attrs(struct module *mod, const struct load_info *info)
@@ -1520,7 +1520,7 @@ static void add_sect_attrs(struct module *mod, const struct load_info *info)
 			+ nloaded * sizeof(sect_attrs->attrs[0]),
 			sizeof(sect_attrs->grp.attrs[0]));
 	size[1] = (nloaded + 1) * sizeof(sect_attrs->grp.attrs[0]);
-	sect_attrs = kzalloc(size[0] + size[1], GFP_KERNEL);
+	sect_attrs = kvzalloc(size[0] + size[1], GFP_KERNEL);
 	if (sect_attrs == NULL)
 		return;