If you compile QEMU with GCC with -fsanitize=address and
-Wstringop-overflow, this causes GCC to produce a false-positive
warning which it does not produce when the sanitizer is not enabled
(and which makes compilation fail if you're using -Werror, as we do
by default for builds from git):
../../hw/net/rtl8139.c: In function ‘rtl8139_io_writeb’:
../../hw/net/rtl8139.c:2264:17: error: writing 8 bytes into a region of size 0
[-Werror=stringop-overflow=]
2264 | memcpy(data_to_checksum, saved_ip_header + 12, 8);
| ^
In file included from ../../hw/net/rtl8139.c:62:
/home/pm215/qemu/include/net/eth.h:50:14: note: at offset [8, 48] into
destination object ‘ip_ver_len’ of size 1
50 | uint8_t ip_ver_len; /* version and header length */
| ^~~~~~~~~~
../../hw/net/rtl8139.c:2192:21: error: writing 8 bytes into a region of size 0
[-Werror=stringop-overflow=]
2192 | memcpy(data_to_checksum, saved_ip_header + 12, 8);
| ^
/home/pm215/qemu/include/net/eth.h:50:14: note: at offset [8, 48] into
destination object ‘ip_ver_len’ of size 1
50 | uint8_t ip_ver_len; /* version and header length */
| ^~~~~~~~~~
../../hw/net/rtl8139.c:2192:21: error: writing 8 bytes into a region of size 0
[-Werror=stringop-overflow=]
2192 | memcpy(data_to_checksum, saved_ip_header + 12, 8);
| ^
/home/pm215/qemu/include/net/eth.h:50:14: note: at offset [8, 48] into
destination object ‘ip_ver_len’ of size 1
50 | uint8_t ip_ver_len; /* version and header length */
| ^~~~~~~~~~
In file included from /home/pm215/qemu/include/system/memory.h:21,
from /home/pm215/qemu/include/hw/pci/pci.h:4,
from /home/pm215/qemu/include/hw/pci/pci_device.h:4,
from ../../hw/net/rtl8139.c:54:
In function ‘stl_he_p’,
inlined from ‘stl_be_p’ at /home/pm215/qemu/include/qemu/bswap.h:371:5,
inlined from ‘rtl8139_cplus_transmit_one’ at ../../hw/net/rtl8139.c:2244:21,
inlined from ‘rtl8139_cplus_transmit’ at ../../hw/net/rtl8139.c:2345:28,
inlined from ‘rtl8139_io_writeb’ at ../../hw/net/rtl8139.c:2728:17:
/home/pm215/qemu/include/qemu/bswap.h:284:5: error: writing 4 bytes into a
region of size 0 [-Werror=stringop-overflow=]
284 | __builtin_memcpy(ptr, &v, sizeof(v));
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/pm215/qemu/include/net/eth.h: In function ‘rtl8139_io_writeb’:
/home/pm215/qemu/include/net/eth.h:50:14: note: at offset [24, 64] into
destination object ‘ip_ver_len’ of size 1
50 | uint8_t ip_ver_len; /* version and header length */
| ^~~~~~~~~~
This has been triaged as a bug in GCC:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114494
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99673
(the sanitizer pass rewrites the IR in a way that conflicts with its
use by the warning pass that runs afterwards).
Since this is the only place in our code where we hit this, work
around it by rewriting the relevant bit of code, and noting in a
comment why we do so.
Cc: [email protected]
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3006
Suggested-by: Daniel P. Berrangé <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>
---
Here's a patch that takes the "just work around this one problem"
approach to the gcc sanitizer compile failure.
On the fence about whether this is worth backporting to stable.
---
hw/net/rtl8139.c | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)
diff --git a/hw/net/rtl8139.c b/hw/net/rtl8139.c
index 2ad6338ebe..eb4dfcfbe0 100644
--- a/hw/net/rtl8139.c
+++ b/hw/net/rtl8139.c
@@ -2092,7 +2092,20 @@ static int rtl8139_cplus_transmit_one(RTL8139State *s)
eth_payload_data = saved_buffer + ETH_HLEN;
eth_payload_len = saved_size - ETH_HLEN;
- ip = (struct ip_header*)eth_payload_data;
+ /*
+ * It would be more natural to write this as
+ * ip = (struct ip_header *)eth_payload_data;
+ * (the IP header is at the start of the ethernet payload).
+ * However, writing it that way triggers a GCC bug where an
+ * interaction between -fsanitize=address and -Wstringop-overflow
+ * results in a false-positive stringop-overflow warning that is
+ * only emitted when the address sanitizer is enabled:
+ * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114494
+ * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99673
+ * So we work around this by writing the expression in an
equivalent
+ * way that doesn't run into this bug.
+ */
+ ip = (struct ip_header *)saved_buffer + ETH_HLEN;
if (IP_HEADER_VERSION(ip) != IP_HEADER_VERSION_4) {
DPRINTF("+++ C+ mode packet has bad IP version %d "
--
2.43.0
