On a musl-based system (Alpine-linux) the basename(3) function is not defined
by including string.h with _GNU_SOURCE defined. However basename(3) could be
defined by including libgen.h.
On musl this is a problem than can lead to a segmentation fault, as I have
experienced. This issue is caused by basename(3) function being implicitly
declared and thus having, implicitly, a return type of int. Which in my case
caused an erroneous sign extension of a pointer leading to a segmentation
fault.
Adding an include for libgen.h sound to me like a proper solution.
Also by doing so the `_GNU_SOURCE` defined is no longer needed.
You can find below details on the issue, on alpine linux (x86_64) running:
$ lspci -s 00:01.0 -v
00:01.0 Host bridge: Advanced Micro Devices, Inc. [AMD] Family 17h-19h PCIe Dummy Host Bridge (rev 01)
Segmentation fault
After debugging, the fault is indirectly caused by this compilation warning:
sysfs.c: In function 'sysfs_fill_info':
sysfs.c:457:53: warning: implicit declaration of function 'basename' [-Wimplicit-function-declaration]
457 | pci_set_property(d, PCI_FILL_IOMMU_GROUP, basename(group_link));
| ^~~~~~~~
sysfs.c:457:53: warning: passing argument 3 of 'pci_set_property' makes pointer from integer without a cast [-Wint-conversion]
457 | pci_set_property(d, PCI_FILL_IOMMU_GROUP, basename(group_link));
| ^~~~~~~~~~~~~~~~~~~~ int
Here is the relevant assembly, dump from gdb:
0x7ffff7f4f072 call *0x5db8(%rip) # call to basename
0x7ffff7f4f078 mov %rbx,%rdi
0x7ffff7f4f07b mov $0x4000,%esi # PCI_FILL_IOMM_GROUP
0x7ffff7f4f080 movslq %eax,%rdx # return value of basename is signed extended from 32bit (eax) to 64bit (rdx)
0x7ffff7f4f083 call 0x7ffff7f4831b # call to pci_set_property
And how the address becomes invalid:
(gdb) x/s 0x7ffff7ffed20 # argument of basename
0x7ffff7ffed20: "/sys/kernel/iommu_groups/0"
(gdb) x/s 0x7ffff7ffed39 # result from basename
0x7ffff7ffed39: "0"
(gdb) x/s 0xfffffffff7ffed39 # after sign extension
0xfffffffff7ffed39: <error: Cannot access memory at address 0xfffffffff7ffed39>
Signed-off-by: Jules Maselbas <jmaselbas@xxxxxxxx>
---
lib/sysfs.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/lib/sysfs.c b/lib/sysfs.c
index 0e763dc..1644e51 100644
--- a/lib/sysfs.c
+++ b/lib/sysfs.c
@@ -9,11 +9,10 @@
* SPDX-License-Identifier: GPL-2.0-or-later
*/
-#define _GNU_SOURCE
-
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
+#include <libgen.h>
#include <stdarg.h>
#include <unistd.h>
#include <errno.h>
--
2.44.0
