Documenting and Automating Collateral Evolutions in Linux Device Drivers
Yoann PadioleauEcole des Mines de Nantes (now at UIUC)
withJulia Lawall and René Rydhof Hansen (DIKU)
Gilles Muller (Ecole des Mines de Nantes)
the Coccinelle project
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The problem: Collateral Evolutions
int bar(int x){
Evolutionin a library
becomes
Can entail lots of Collateral Evolutions (CE) in clients
foo(1);bar(1);
foo(foo(2));bar(bar(2));
int foo(int x){lib.c
client1.c client2.c
clientn.c
foo(2);bar(2);
if(foo(3)) {if(bar(3)) {
Legend:beforeafter
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The problem: Collateral Evolutions
int bar(int x, int y){
Evolutionin a library
becomes
Can entail lots of Collateral Evolutions (CE) in clients
foo(1);bar(1,?);
foo(foo(2));bar(bar(2,?),?);
int foo(int x){lib.c
client1.c client2.c
clientn.c
foo(2);bar(2,?);
if(foo(3)) {if(bar(3,?)) {
Legend:beforeafter
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Our target: Linux device drivers Many libraries and many clients:
Lots of driver support libraries: one per device type, one per bus (pci library, sound library, …)
Lots of device specific code: Drivers make up more than 50% of Linux
Many evolutions and collateral evolutions [Eurosys’06] 1200 evolutions in Linux 2.6 For each evolution, lots of collateral evolutions Some collateral evolutions affect over 400 files
at over 1000 code sites
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Our goal Currently, Collateral Evolutions in Linux are done
nearly manually: Difficult Time consuming Error prone
The highly concurrent and distributed nature of the Linux development process makes it even worse:
Patches that miss code sites (because of newly introduced sites and newly introduced drivers)
Out of date patches, conflicting patches Drivers outside the Linux source tree are not updated Misunderstandings
Need a tool to document and automate Collateral Evolutions
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Taxonomy of transformations Taxonomy of evolutions (library code):
add parameter, split data structure, change protocol sequencing, change return type, add error code, etc
Taxonomy of collateral evolutions (client code)?
Very wide variety of program transformations, affecting wide variety of C and CPP constructs
Often depends on context, e.g. for add argument the new value must be constructed from enclosing code
Note that not necesseraly semantic preservingCan not be done by current refactoring tools (more than just renaming entities). Need a flexible tool.
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Complex Collateral Evolutions (2.5.71)
int a_proc_info(int x ) { scsi *y; ... y = scsi_get(); if(!y) { ... return -1; } ... scsi_put(y);
... }
Delete calls
to libraryDelete error
checking code
From local var to
parameter
Legend:beforeafter
,scsi *y;
Evolution: scsi_get()/scsi_put() dropped from SCSI library Collateral evolutions: SCSI resource now passed directly to
proc_info callback functions via a new parameter
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Our idea
int a_proc_info(int x ,scsi *y ) { scsi *y; ... y = scsi_get(); if(!y) { ... return -1; } ... scsi_put(y);
... }
The example How to specify
the required program transformation ?
In what programming language ?
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Our idea: Semantic Patches
int a_proc_info(int x+ ,scsi *y ) {- scsi *y; ...- y = scsi_get();- if(!y) { ... return -1; } ...- scsi_put(y);
... }
function a_proc_info; identifier x,y;
@@
@@
the ‘...’ operator
modifiersDeclarative
language
Patch-like syntax
metavariablereferencesTransform if
everything
“matches”
metavariable declarations
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Affected Linux driver codeint s53c700_info(int limit) { char *buf; scsi *sc; sc = scsi_get(); if(!sc) { printk(“error”); return -1; } wd7000_setup(sc); PRINTP(“val=%d”, sc->field+limit); scsi_put(sc); return 0;}
int nsp_proc_info(int lim) { scsi *host; host = scsi_get(); if(!host) { printk(“nsp_error”); return -1; } SPRINTF(“NINJASCSI=%d”, host->base); scsi_put(host); return 0;}
drivers/scsi/53c700.c
Similar, but not identical
drivers/scsi/pcmcia/nsp_cs.c
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Applying the semantic patch
@@function a_proc_info; identifier x,y;@@ int a_proc_info(int x+ ,scsi *y ) {- scsi *y; ...- y = scsi_get();- if(!y) { ... return -1; } ...- scsi_put(y);
... }
proc_info.sp
int nsp_proc_info(int lim) { scsi *host; host = scsi_get(); if(!host) { printk(“nsp_error”); return -1; } SPRINTF(“NINJASCSI=%d”, host->base); scsi_put(host); return 0;}
int s53c700_info(int limit) { char *buf; scsi *sc; sc = scsi_get(); if(!sc) { printk(“error”); return -1; } wd7000_setup(sc); PRINTP(“val=%d”, sc->field+limit); scsi_put(sc); return 0;}
$ spatch *.c < proc_info.sp
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Applying the semantic patch
@@function a_proc_info; identifier x,y;@@ int a_proc_info(int x+ ,scsi *y ) {- scsi *y; ...- y = scsi_get();- if(!y) { ... return -1; } ...- scsi_put(y);
... }
int s53c700_info(int limit, scsi *sc) { char *buf; wd7000_setup(sc); PRINTP(“val=%d”, sc->field+limit); return 0;}
proc_info.sp
int nsp_proc_info(int lim, scsi *host) { SPRINTF(“NINJASCSI=%d”, host->base); return 0;}
$ spatch *.c < proc_info.sp
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SmPL: Semantic Patch Language A single small semantic patch can modify
hundreds of files, at thousands of code sites The features of SmPL make a semantic patch
generic. Abstract away irrelevant details: Differences in spacing, indentation, and comments Choice of the names given to variables
(metavariables) Irrelevant code (‘...’, control flow oriented) Other variations in coding style (isomorphisms)
e.g. if(!y) ≡ if(y==NULL) ≡ if(NULL==y)
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Sequences and the ‘…’ operator
One ‘-’ line can erase multiple lines
1 y = scsi_get();2 if(exp) {3 scsi_put(y);4 return -1;5 }
6 printf(“%d”,y->f); 7 scsi_put(y);8 return 0;
- y = scsi_get(); ...- scsi_put(y);
C file Semantic patch
Control-flow graph(CFG) of C file 1
2
3
8
6
4
7
exit
path 1:
path 2:
“. . .” means for all subsequent paths
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Isomorphisms, C equivalences Examples:
Boolean : X == NULL !X NULL == X
Control : if(E)S1 else S2 if(!E) S2 else S1 Pointer : E->field *E.field etc.
How to specify isomorphisms ?
Reuses SmPL syntax
@@ expression *X; @@X == NULL <=> !X <=> NULL == X
How does it work ?
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The transformation engine architectureParse C file Parse Semantic Patch
Translate to CFG
Translate to extended CTL
Expand isomorphisms
Match CTL against CFG using
a model checking algorithm
Modify matched code
Unparse
Computational Tree Logic
[Clark86] with extra features
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CTL and Model checking Model checking a CTL formula against a
model answers just yes/no (with counter example).
We do program transformations, not just “pattern checking”. Need: Bind metavariables and remember their value Remember where we have matched sub-
formulas We have extended CTL : existential
variables and program transformation annotations
@@ exp X,Y;@@ f(X); ...- g(Y);+ g(X,Y);
9X.f(X);Æ AX A[true U 9v.9Y.g-(-Y-)-;-+g(X,Y);
v
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Other issues Need to produce readable code
Keep space, indentation, comments Keep CPP instructions as-is. Also programmer
may want to transform some #define,iterator macros (e.g. list_for_each)
Interactive engine, partial match Isomorphisms
Rewriting the Semantic patch (not the C code), Generate disjunctions
Very different from most other C tools
60 000 lines of OCaml code
Evaluation
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Experiments Methodology
Detect past collateral evolutions in Linux 2.5 and 2.6 using patchparse tool [Eurosys’06]
Select representative ones Test suite of over 60 CEs
Study them and write corresponding semantic patches
Note: we are not kernel developers
Going "back to the future". Compare: what Linux programers did manually what spatch, given our SPs, does automatically
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Test suite 20 Complex CEs : mistakes done by
the programmers In each case 1-16 errors or misses
23 Mega CEs : affect over 100 sites Up to 40 people for up to two years
26 “typical” CEs: The whole set of CEs affecting a
“typical” (median) directory from 2.6.12 to 2.6.20More than 5800 driver files
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Results Our SPs are on average 106 lines long SPs often 100 times smaller than “human-made”
patches. A measure of time saved: Not doing manually the CE on all the drivers Not reading and reviewing big patches, for people with
drivers outside source tree Overall: correct and complete automated
transformation for 93% of files Problems on the remaining 7%: We miss code sites
CPP issues, lack of isomorphisms (data-flow and inter-procedural)
We are not kernel developers … don’t know how to specify No false positives, just false negatives
Average processing time of 0.7s per relevant fileSometimes the tool was right and human wrong
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Impact on the Linux kernel We also wrote some SPs for current collateral
evolutions (looking at linux kernel mailing lists) use DIV_ROUND_UP, BUG_ON, FIELD_SIZE convert kmalloc-memset to kzalloc Total diffstat: 154 files changed, 203 insertions(+), 375
deletions(-) We wrote other SPs, for “bug-fixing” (good side
effects of our tool) Add missing put functions (reference counting) Drop unnecessary put functions (reference counting) Remove unused variables Total diffstat: 111 files changed, 340 insertions(+), 355
deletions(-)
Accepted in latest Linux kernel
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Future work Are semantic patches and spatch useful
Only for Linux device drivers? Only for Linux programmers? Only for collateral evolutions program
transformations? Only for program transformations?
Our thesis: We don’t think so. But first device driver CEs are an important
problem! All software evolves. Software libraries are
more and more important, and have more and more clients
We may also help that software, those libraries
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Related work Refactoring
CatchUp[ICSE’05], tool-based, replay refactorings from Eclipse
JunGL[ICSE’06], language-based, but based on ML, less Linux-programmer friendly
Program transformation engines Stratego[04]
C front-ends CIL[CC’02]
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Conclusion Collateral Evolution is an important
problem, especially in Linux device drivers SmPL: a declarative language to specify
collateral evolutions Looks like a patch; fits with Linux
programmers’ habits But takes into account the semantics of C
hence the name Semantic Patches A transformation engine to automate
collateral evolutions based on model checking technology.
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Thank you You can download our tool, spatch, at
http://www.emn.fr/x-info/coccinelle Questions ?
