Launchpad has imported 6 comments from the remote bug at https://sourceware.org/bugzilla/show_bug.cgi?id=25497.
If you reply to an imported comment from within Launchpad, your comment will be sent to the remote bug automatically. Read more about Launchpad's inter-bugtracker facilities at https://help.launchpad.net/InterBugTracking. ------------------------------------------------------------------------ On 2020-02-03T16:12:07+00:00 Konstantin wrote: When fseek called, it in turn calls lseek (as expected), and then calls read() over the skipped range (as not expected). In the best case, it's a waste of CPU and IO resources. In the worst case, this causes an application that tried to skip too big range to just hang on fseek(). This is a follow up to discussion at https://sourceware.org/ml/libc- help/2020-01/threads.html#00046 # Steps to reproduce (in terms of terminal commands) $ cat test.c #include <fcntl.h> #include <stdio.h> int main() { FILE* f = fopen("/tmp/test.c", "r"); if (!f) perror(""); fseek(f, 30, SEEK_SET); } $ gcc test.c -o a $ strace ./a 2>&1 | tail mprotect(0x7fd2c36c1000, 4096, PROT_READ) = 0 munmap(0x7fd2c3628000, 451693) = 0 brk(NULL) = 0x557c9e900000 brk(0x557c9e921000) = 0x557c9e921000 openat(AT_FDCWD, "/tmp/test.c", O_RDONLY) = 3 fstat(3, {st_mode=S_IFREG|0644, st_size=155, ...}) = 0 lseek(3, 0, SEEK_SET) = 0 read(3, "#include <fcntl.h>\n#include <s", 30) = 30 exit_group(0) = ? +++ exited with ## Expected There's no read() call after lseek() ## Actual Both lseek() and read() are called. Reply at: https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1861776/comments/0 ------------------------------------------------------------------------ On 2020-02-03T16:19:43+00:00 Carlos-0 wrote: I'm not sure what the consequences are for optimizing away the read as part of the FILE buffer management. That is the question that would need to be answered here before we could do something like this. Reply at: https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1861776/comments/1 ------------------------------------------------------------------------ On 2020-02-03T17:11:03+00:00 Andreas Schwab wrote: The read is required to sychronize the underlying file position, while keeping the stdio buffer aligned on a block boundary. Reply at: https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1861776/comments/2 ------------------------------------------------------------------------ On 2020-02-03T18:05:01+00:00 Konstantin wrote: (In reply to Andreas Schwab from comment #2) > The read is required to sychronize the underlying file position, while > keeping the stdio buffer aligned on a block boundary. Though I don't know why it's necessary, but would it be possible in this case to at least only read just one block, that is the last block before the position a program is trying to set with fseek()? So at least, when a program tries to do fseek(…,0x80000000, SEEK_SET), it wouldn't hang on fseek trying to read half a terabyte of data. Reply at: https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1861776/comments/3 ------------------------------------------------------------------------ On 2020-02-03T19:47:30+00:00 Andreas Schwab wrote: Where do you see it reading more than one block? Reply at: https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1861776/comments/4 ------------------------------------------------------------------------ On 2020-02-03T22:47:23+00:00 Konstantin wrote: (In reply to Andreas Schwab from comment #4) > Where do you see it reading more than one block? Oh, I stand corrected, on glibc 2.30 this is no longer reproducible. Though it's reproducible on glibc 2.27, just 3 versions ago. Reproducing that simply requires one to run something like `sudo hexdump -C /dev/sda -s 0xa8000f9000 -n 1`: if it hangs, it's because `fseek()` hexdump is using tries to read 0xa8000f9000 amount of data. Reply at: https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1861776/comments/5 ** Changed in: glibc Status: Unknown => New ** Changed in: glibc Importance: Unknown => Medium -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1861776 Title: fseek(…, …, SEEK_SET) causes reading over the skipped range To manage notifications about this bug go to: https://bugs.launchpad.net/glibc/+bug/1861776/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
