- And how it can help fight linux kernel ABI breakages
Samikshan Bairagya
By,
{#,@}samikshan
Introducing 'spartakus'
Content available under Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)

Topics to cover
●
What is kernel ABI
●
Why does kABI matter
●
How to track kABI breakages
●
Static code analysis using genksyms
●
Issues with genksyms
●
Semantic code analysis using sparse (spartakus)

What is ABI?
●
ABI == Application Binary Interface
●
Like API, but at binary level
●
Interface for application to use external library,
OS at binary level.
●
If ABI does not change, applications compiled
against that ABI do not need recompilation

Linux kernel ABI?
Binary interface exported by linux kernel and
accompanying kernel modules

EXPORT_SYMBOL
~/r/k/linux-3.14> grep -Rn "EXPORT_SYMBOL" .
./kernel/async.c:303:EXPORT_SYMBOL_GPL(async_synchronize_cookie_domain);
./kernel/async.c:316:EXPORT_SYMBOL_GPL(async_synchronize_cookie);
./kernel/gcov/base.c:52:EXPORT_SYMBOL(__gcov_init);
./kernel/gcov/base.c:62:EXPORT_SYMBOL(__gcov_flush);
./kernel/gcov/base.c:68:EXPORT_SYMBOL(__gcov_merge_add);
./kernel/gcov/base.c:74:EXPORT_SYMBOL(__gcov_merge_single);
./kernel/gcov/base.c:80:EXPORT_SYMBOL(__gcov_merge_delta);
./kernel/gcov/base.c:86:EXPORT_SYMBOL(__gcov_merge_ior);
./kernel/tracepoint.c:395:EXPORT_SYMBOL_GPL(tracepoint_probe_register);
./kernel/tracepoint.c:439:EXPORT_SYMBOL_GPL(tracepoint_probe_unregister);

Why strive for kABI stability
● kABI instability does not provide a welcoming
third party environment to the hardware
manufacturers
● Stable kABI means
– Third party modules don't break and recompilation
against the changed ABI won't be needed
– Specially important for enterprise Linux vendors

How to track kernel ABI
● ABI checksum information (modversions) for
exported kernel symbols
– Information stored in Module.symvers file
– Generated during kernel build process using genksyms

Module.symvers
$ cat /usr/src/kernels/4.0.4-303.fc22.x86_64/Module.symvers
0x00000000 iscsi_host_add drivers/scsi/libiscsi EXPORT_SYMBOL_GPL
0x00000000 mpt_deregister drivers/message/fusion/mptbase EXPORT_SYMBOL
0x00000000 tm6000_set_reg_mask drivers/media/usb/tm6000/tm6000
EXPORT_SYMBOL_GPL
0x00000000 kvm_get_cs_db_l_bits arch/x86/kvm/kvm
EXPORT_SYMBOL_GPL
0x00000000 ipv6_chk_custom_prefix vmlinux EXPORT_SYMBOL
0x00000000 sata_pmp_error_handler vmlinux EXPORT_SYMBOL_GPL
0x00000000 __cleancache_invalidate_page vmlinux EXPORT_SYMBOL
0x00000000 pcmcia_reset_card vmlinux EXPORT_SYMBOL
0x00000000 unregister_vt_notifier vmlinux EXPORT_SYMBOL_GPL
0x00000000 kmem_cache_alloc vmlinux EXPORT_SYMBOL
0x00000000 replace_page_cache_page vmlinux EXPORT_SYMBOL_GPL
0x00000000 __cond_resched_softirq vmlinux EXPORT_SYMBOL

genksyms
●
Generates checksums for exported kernel
symbols during build process
●
Static code analysis
●
Part of the linux kernel source code
●
tools/genksyms/

genksyms - issues
Strict string processing of kernel symbol
definition prototypes leads to false positives at
times

genksyms generates different checksums for
structs/unions for declarations which are
semantically same
genksyms – issues (contd)

genksyms – issues (contd)
● 2 semantically same
struct declarations
should not lead to a
change in kABI
● But due to strict string
processing in genksyms,
checksums generated
for these 2 declarations
are different
struct list_head {
struct list_head *next,
*prev;
};
struct list_head {
struct list_head *next;
struct list_head *prev;
};

Misleading checksums generated for variables specified as
unsigned/signed of no particular data type.
●
Both declarations above are semantically similar
●
genksyms fails to identify this
genksyms – issues (contd)
'unsigned foo' and 'unsigned int foo'

Possible solution?
spartakus - Using sparse for semantic processing
of linux source code

Using spartakus to generate checksums through semantic
processing of source code eliminates such issues
●
‘unsigned foo’ is converted to ‘unsigned int foo’ and
then processed to get the corresponding checksum
●
Semantically similar struct/union declarations are
treated as same and different checksums are not
generated

Wait, but what is spartakus?
●
A fork of sparse
– 'semantic parser of source files
– creates a semantic parse tree for further
analysis
●
Provides additional binary 'check_kabi'
– Generates checksums for exported kernel
symbols like genksyms during kernel build
time

Is spartakus ready yet?

$ git clone https://github.com/samikshan/spartakus.git
– Contribute
– License: GPLv2

Some URLs for reference:
●
Sparse:
– http://kernelnewbies.org/Sparse
●
Kernel ABI
– http://kernelnewbies.org/ABI
– http://www.jonmasters.org/blog/2007/06/16/ludi
crously-technical-kernel-abi-tracking/

Questions?
Content available under Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)
samikshan@gmail.com
Contact:

Thanks