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List: kde-core-devel
Subject: Patch for mmap problem in KConfigBackEnd, Bug #72586
From: "Dr. Juergen Pfennig" <info () j-pfennig ! de>
Date: 2004-01-15 13:20:16
Message-ID: 200401151420.17108.info () j-pfennig ! de
[Download RAW message or body]
Hello whoever,
I send you a patch to avoid that Konqueror hangs and Syslogd quickly fills
your harddisk with error messages. The problem occurs with mmap() and files
that are accessed via smb (or nfs) and are not accessible due to ownership
and protection on the server side. See the remark concerning nfs in 'man 2
access'. Typically the problem shows up with konqueror and the .directory
file. Lots of lines containing just the work "SIGBUS" are written to stderr
and konqueror hangs.
About the current implementation in kconfigbackend.cpp: mmap() is used always
to read conf files. Interrestingly the author was aware of a SIGBUS problem
and tried to recover from the SIGBUS. BUT THIS DOES NOT WORK. Maybe it never
worked? Or maybe it's a compiler optimization problem.
About the patch: as mmap() may return unaccessible memory, the use of mmap is
restricted now to large files. In any case a classical read() syscall is used
either to read an entiere small conf file, or to probe if the (large) file is
readable. If the read succeeds for a large file, mmap is used as before. mmap
is not trivial for the kernel, maybe this patch also gives a little speed
improvement. The patch is against today's HEAD (3.1.94 plus updates).
I also attach a little test program that can be used to play with access() and
mmap() if you don't trust me. My kernel is a 2.4.21 from Mantel/SuSE. I am
using a P4 and the problem occurred also in older Versions of KDE.
Yours Jürgen
["jpf.patch" (text/x-diff)]
--- kconfigbackend.cpp 2004-01-15 13:16:49.000000000 +0100
+++ kconfigbackend.cpp 2004-01-15 13:11:06.000000000 +0100
@@ -367,13 +367,14 @@
#ifdef HAVE_MMAP
#ifdef SIGBUS
-static const char **mmap_pEof;
static void mmap_sigbus_handler(int)
{
- *mmap_pEof = 0;
- write(2, "SIGBUS\n", 7);
- signal(SIGBUS, mmap_sigbus_handler);
+ const char* pMsg = "FATAL: KConfigBackend "
+ "cannot recover from SIGBUS error on mmap'ed config file\n";
+ write(2, pMsg, strlen(pMsg));
+ // we must commit suicide, otherwise we an infinite loop
+ _exit(2);
}
#endif
#endif
@@ -399,33 +400,58 @@
unsigned int ll = localeString.length();
-#ifdef HAVE_MMAP
- const char *map = ( const char* ) mmap(0, rFile.size(), PROT_READ, MAP_PRIVATE,
- rFile.handle(), 0);
+ bool fileOptionImmutable = false;
+ bool groupOptionImmutable = false;
+ bool groupSkip = false;
- if (map)
- {
- s = map;
+ // mmap is fast for large config files but can fail to return readable
+ // memory for smb or nfs mounts (see access() manpage). The fastest
+ // workaround is to probe files using read(). Small files are read
+ // directly, large files are left to mmap().
+
+ char sBuffer[1024];
+ int iOK;
+ if(rFile.size() <= sizeof(sBuffer))
+ { s = sBuffer;
eof = s + rFile.size();
-
-#ifdef SIGBUS
- mmap_pEof = &eof;
- old_sighandler = signal(SIGBUS, mmap_sigbus_handler);
-#endif
+ iOK = read(rFile.handle(), sBuffer, rFile.size());
}
else
+ { s = eof = 0;
+ iOK = read(rFile.handle(), sBuffer, 1);
+ }
+ if(iOK < 0)
+ { // kdDebug cannot be used here?
+ fprintf(stderr, "KConfigBackend cannot read file: %s\n", rFile.name().latin1());
+ return;
+ }
+
+#ifdef HAVE_MMAP
+ const char *map = 0; // we need it for munmap()
#endif
+ if(s == 0)
{
- rFile.at(0);
- data = rFile.readAll();
- s = data.data();
- eof = s + data.size();
+#ifdef HAVE_MMAP
+ map = (const char*)mmap(0, rFile.size(), PROT_READ, MAP_PRIVATE,
+ rFile.handle(), 0);
+ if(map)
+ { s = map;
+ eof = s + rFile.size();
+ // SIGBUS cannot be used to recover invalid memory access!
+#ifdef SIGBUS
+ old_sighandler = signal(SIGBUS, mmap_sigbus_handler);
+#endif
+ }
+ else
+#endif
+ { // use qt if mmap is not availlable
+ rFile.at(0);
+ data = rFile.readAll();
+ s = data.data();
+ eof = s + data.size();
+ }
}
- bool fileOptionImmutable = false;
- bool groupOptionImmutable = false;
- bool groupSkip = false;
-
int line = 0;
for(; s < eof; s++)
{
@@ -447,7 +473,7 @@
if (*s == '[') //group
{
// In a group [[ and ]] have a special meaning
- while ((s < eof) && (*s != '\n'))
+ while ((s < eof) && (*s != '\n'))
{
if (*s == ']')
{
["smbproblem.cpp" (text/x-c++src)]
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/mman.h>
#include <signal.h>
char* pX;
void sighandler(int iSig)
{
printf("sighandler %d\n", iSig);
pX = "";
}
int main()
{
const char* pszFile = "/Filme/Server/.directory";
printf("testing: %s\n", pszFile);
// access won't help ...
int ia = access(pszFile, R_OK);
printf("access returns %x\n", ia);
// open doesn't fail ...
int io = open(pszFile, O_RDONLY);
printf("open returns %x\n", io);
// fstat works ...
struct stat st;
int is = fstat(io, &st);
printf("fstat() returns %x mode %x uid %d gid %d\n",
is, st.st_mode, st.st_uid, st.st_gid);
// read fails ...
char buff[128];
int ir = read(io, buff, st.st_size);
if(ir >= 0) printf("read() returns %d\n", ir);
else perror("read() status");
// mmap succeeds ...
void* pMap = mmap(0, st.st_size, PROT_READ, MAP_PRIVATE, io, 0);
if(pMap) printf("mmap() returns %x\n", pMap);
else perror("mmap() status");
// write using the mmap memory fails
int iw = write(0, pMap, 1);
if(iw >= 0) printf("write() returns %x\n", iw);
else perror("write() status");
// THIS IS CURRENTLY IN KDE AND IS WRONG!
// signal(SIGBUS, sighandler);
// printf("data %x\n", pX[0]);
return 0;
}
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