struct flock is posix defined locking mechanism on *nix platform
Example usage (copied from https://gavv.github.io/articles/file-locks/) :
#include <fcntl.h>
struct flock fl;
memset(&fl, 0, sizeof(fl));
// lock in shared mode
fl.l_type = F_RDLCK;
// lock entire file
fl.l_whence = SEEK_SET; // offset base is start of the file
fl.l_start = 0; // starting offset is zero
fl.l_len = 0; // len is zero, which is a special value representing end
// of file (no matter how large the file grows in future)
fl.l_pid = 0; // F_SETLK(W) ignores it; F_OFD_SETLK(W) requires it to be zero
// F_SETLKW specifies blocking mode
if (fcntl(fd, F_SETLKW, &fl) == -1) {
exit(1);
}
// atomically upgrade shared lock to exclusive lock, but only
// for bytes in range [10; 15)
//
// after this call, the process will hold three lock regions:
// [0; 10) - shared lock
// [10; 15) - exclusive lock
// [15; SEEK_END) - shared lock
fl.l_type = F_WRLCK;
fl.l_start = 10;
fl.l_len = 5;
// F_SETLKW specifies non-blocking mode
if (fcntl(fd, F_SETLK, &fl) == -1) {
exit(1);
}
// release lock for bytes in range [10; 15)
fl.l_type = F_UNLCK;
if (fcntl(fd, F_SETLK, &fl) == -1) {
exit(1);
}
// close file and release locks for all regions
// remember that locks are released when process calls close()
// on any descriptor for a lock file
close(fd);
It is fact that ld.so has libc statially linked into it.
Normally we wouldn't need ld.so functionality once the program is
finalyl loaded, but with the next few patches, we will have dlopen which
will reuse the same ld.so functionality.
The problem is that it seams that huge part of the code is possible not
referntially transparent. That is, it is not impossible that some of the
functions have internals states. So when using the struct Linker that is
initialized by ld.so's copy of libc. we must access it using the same
copy even if both copies are identical.
For example in dlopen if you do linker.load_library(..). That would
segfault because it is using the function from libc not ld.so
So I don't truly undestand why should this be needed, but after long
hours of being stuck I thought maybe.. maybe that is the issue and
indeed it turned out to be so.
I attempted fixing this issue before at 43fbaf99. Although it did work,
it worked wrong, and it was just consistently working (but in wrong way)
until it didn't.
Since this is (hopefully) the real fix, I will try to explain exactly
what is going on.
This is explaination by example:
our TLS is memory of size 0x1000 starting at 0x7ffff6c50000,
but the real size is 0x000068 so we have padding stored at master.offset
= 0xf98
Now our symbol looks as follows
Offset Type Sym. Value Name
000000432b20 R_X86_64_DTPOFF64 0000000000000058 errno
The old code did 0x7ffff6c50000 + 0xf98 + 000000432b20 which is
obviosly overflowing the memory and wrong.
The right way 0x7ffff6c50000 + 0xf98 + 0000000000000058.
THe Tls base part and offset are added at __tls_get_addr function.
What is left is storing the 0x58 at the relocation address. The problem
is that we don't have 0x58, but we have (binary base + 0x58) in global
symbol table and binary base so what we store is the (binarybase + 0x58
- binary base).
I hope this does turn out to be wrong.
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