Re: malloc() call somehow calling the rtld malloc() implementaion

On Fri, 27 Jan 2017 10:47:20 -0800 (PST)
Don Lewis <truckman_at_FreeBSD.org> wrote:

> On 27 Jan, Alexander Kabaev wrote:
> > On Fri, 27 Jan 2017 00:31:30 -0800 (PST)
> > Don Lewis <truckman_at_FreeBSD.org> wrote:  
> 
> >> If I create a simple test program that calls malloc() and set a
> >> breakpoint in malloc(), the breakpoint gets set in the rtld
> >> version, but the the libc version of malloc is what gets called.
> >> 
> >> What the heck is going on here, and how can I fix it?
> >>   
> > 
> > rtld on my system does not have malloc exposed as dynamic symbol, it
> > cannot possibly be used for symbol resolution by any outside
> > module.  
> 
> Same here, but gdb at least seems to find it anyway.
> 
> 12.0-CURRENT r311765M
> 
> %nm /libexec/ld-elf.so.1
> nm: /libexec/ld-elf.so.1: no symbols
> 
> %elfdump -s /libexec/ld-elf.so.1 | grep st_name | sort
> 	st_name: 
> 	st_name: 
> 	st_name: FBSD_1.0
> 	st_name: FBSD_1.1
> 	st_name: FBSD_1.2
> 	st_name: FBSD_1.3
> 	st_name: FBSD_1.4
> 	st_name: FBSD_1.5
> 	st_name: FBSDprivate_1.0
> 	st_name: __tls_get_addr
> 	st_name: _r_debug_postinit
> 	st_name: _rtld_addr_phdr
> 	st_name: _rtld_allocate_tls
> 	st_name: _rtld_atfork_post
> 	st_name: _rtld_atfork_pre
> 	st_name: _rtld_error
> 	st_name: _rtld_free_tls
> 	st_name: _rtld_get_stack_prot
> 	st_name: _rtld_is_dlopened
> 	st_name: _rtld_thread_init
> 	st_name: dl_iterate_phdr
> 	st_name: dladdr
> 	st_name: dlclose
> 	st_name: dlerror
> 	st_name: dlfunc
> 	st_name: dlinfo
> 	st_name: dllockinit
> 	st_name: dlopen
> 	st_name: dlsym
> 	st_name: dlvsym
> 	st_name: fdlopen
> 	st_name: r_debug_state
> 
> %cd /tmp
> zipper:/tmp 508%cat malloctest.c 
> #include <stdlib.h>
> volatile void *p;
> int
> main(void) {
> 	p = malloc(16);
> }
> %cc -g malloctest.c
> %gdb a.out
> GNU gdb 6.1.1 [FreeBSD]
> Copyright 2004 Free Software Foundation, Inc.
> GDB is free software, covered by the GNU General Public License, and
> you are welcome to change it and/or distribute copies of it under
> certain conditions. Type "show copying" to see the conditions.
> There is absolutely no warranty for GDB.  Type "show warranty" for
> details. This GDB was configured as "amd64-marcel-freebsd"...
> (gdb) break main
> Breakpoint 1 at 0x40076b: file malloctest.c, line 5.
> (gdb) run
> Starting program: /tmp/a.out 
> 
> Breakpoint 1, main () at malloctest.c:5
> 5		p = malloc(16);
> Current language:  auto; currently minimal
> (gdb) break malloc
> Breakpoint 2 at 0x80060e9a4: file /usr/src/libexec/rtld-elf/malloc.c,
> line 163. (gdb) cont
> Continuing.
> 
> Program exited normally.
> (gdb) quit
> 
> Ports gdb finds both the rtld malloc() and the libc malloc():
> 
> %/usr/local/bin/gdb a.out
> GNU gdb (GDB) 7.12 [GDB v7.12 for FreeBSD]
> Copyright (C) 2016 Free Software Foundation, Inc.
> License GPLv3+: GNU GPL version 3 or later
> <http://gnu.org/licenses/gpl.html> This is free software: you are
> free to change and redistribute it. There is NO WARRANTY, to the
> extent permitted by law.  Type "show copying" and "show warranty" for
> details. This GDB was configured as "x86_64-portbld-freebsd12.0".
> Type "show configuration" for configuration details.
> For bug reporting instructions, please see:
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> Find the GDB manual and other documentation resources online at:
> <http://www.gnu.org/software/gdb/documentation/>.
> For help, type "help".
> Type "apropos word" to search for commands related to "word"...
> Reading symbols from a.out...done.
> (gdb) break main
> Breakpoint 1 at 0x40076b: file malloctest.c, line 5.
> (gdb) run
> Starting program: /tmp/a.out 
> 
> Breakpoint 1, main () at malloctest.c:5
> 5		p = malloc(16);
> (gdb) break malloc
> Breakpoint 2 at 0x80060e9a4: malloc. (2 locations)
> (gdb) cont
> Continuing.
> 
> Breakpoint 2, __malloc (size=16) at jemalloc_jemalloc.c:1636
> 1636		size_t usize JEMALLOC_CC_SILENCE_INIT(0);

gdb looks into debugging info for symbols in debug information and so
has greater visibility. Symbols gdb sees play no role in dynasmic
building, so rtld malloc somehow replacing the malloc from the program
as you suspected is really impossible.

kib_at_ has a better idea that is worth investigating further: is the
address on which idlc traps corresponds to TLS storage? tls alloc calls
are supposed to respect the tls section alignment though.

-- 
Alexander Kabaev