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ld.so.8: LD_BIND_NOT has effect only for function symbols 

As far as I can tell from testing and a short read of the glibc
source code, LD_BIND_NOT has an effect only for function symbols.
This is consistent also with the Solaris documentation, which
says:

    When the runtime linker performs a function relocation, it
    rewrites data associated with the functions .plt so that any
    subsequent calls will go directly to the function. The
    environment variable LD_BIND_NOT can be set to any value to
    prevent this data update. By using this variable together
    with the debugging request for detailed bindings, you can
    get a complete runtime account of all function binding.

Test case:

$ cat prog.c

int
main(int argc, char *argv[])
{
    for (;;) {
	sleep(1);
	optind = 0;
    }

    exit(EXIT_SUCCESS);
}
$ cc prog.c

And then in the run below, in each loop, we see repeated resolutions
only for the function symbol, sleep() (and not for optind).

$ LD_BIND_NOT=1 LD_DEBUG=symbols,bindings ./a.out
...
     30481:	transferring control: ./t
     30481:
     30481:	symbol=sleep;  lookup in file=./t [0]
     30481:	symbol=sleep;  lookup in file=/lib64/libc.so.6 [0]
     30481:	binding file ./t [0] to /lib64/libc.so.6 [0]: normal symbol `sleep' [GLIBC_2.2.5]
     30481:	symbol=sleep;  lookup in file=./t [0]
     30481:	symbol=sleep;  lookup in file=/lib64/libc.so.6 [0]
     30481:	binding file ./t [0] to /lib64/libc.so.6 [0]: normal symbol `sleep' [GLIBC_2.2.5]
     30481:	symbol=sleep;  lookup in file=./t [0]
     30481:	symbol=sleep;  lookup in file=/lib64/libc.so.6 [0]
     30481:	binding file ./t [0] to /lib64/libc.so.6 [0]: normal symbol `sleep' [GLIBC_2.2.5]
     30481:	symbol=sleep;  lookup in file=./t [0]
     30481:	symbol=sleep;  lookup in file=/lib64/libc.so.6 [0]
     30481:	binding file ./t [0] to /lib64/libc.so.6 [0]: normal symbol `sleep' [GLIBC_2.2.5]
     30481:	symbol=sleep;  lookup in file=./t [0]
     30481:	symbol=sleep;  lookup in file=/lib64/libc.so.6 [0]
     30481:	binding file ./t [0] to /lib64/libc.so.6 [0]: normal symbol `sleep' [GLIBC_2.2.5]
     30481:	symbol=sleep;  lookup in file=./t [0]
     30481:	symbol=sleep;  lookup in file=/lib64/libc.so.6 [0]
     30481:	binding file ./t [0] to /lib64/libc.so.6 [0]: normal symbol `sleep' [GLIBC_2.2.5]

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2017-01-16 09:32:14 +13:00
parent 534267c408
commit a3a72ad210
1 changed files with 1 additions and 1 deletions
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2
man8/ld.so.8
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@ -354,7 +354,7 @@ in the chapter
.BR LD_BIND_NOT " (since glibc 2.1.95)"
If this environment variable is set to a nonempty string,
do not update the GOT (global offset table) and PLT (procedure linkage table)
after resolving a symbol.
after resolving a function symbol.
.TP
.BR LD_DEBUG " (since glibc 2.1)"
Output verbose debugging information about the dynamic linker.