arm-none-eabi-nm: some symbols are not related to any source file

extern int g_my_externd_global;
int g_my_externd_global;
int g_my_private_global;
int main(void)
{
	g_my_externd_global=1;
	g_my_private_global=1;
	return 0;
}

max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-gcc -g3 -gdwarf-4 -mthumb -mcpu=cortex-m4 -nostdlib -ffunction-sections -fdata-sections -O1 -fno-common -c main.c -o example-O1.o 
max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-nm -s -l example-O1.o 
00000000 B g_my_externd_global
00000000 B g_my_private_global	/home/max/Dropbox/4202/prog/test-dwarf/main.c:3
00000000 T main	/home/max/Dropbox/4202/prog/test-dwarf/main.c:4
00000000 n wm4.0.771cfe8abdd6d09387c8f60d9b1eb4ff

max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-gcc -g3 -gdwarf-4 -mthumb -mcpu=cortex-m4 -nostdlib -ffunction-sections -fdata-sections -O0 -fno-common -c main.c -o example-O0.o 
max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-nm -s -l example-O0.o 
00000000 B g_my_externd_global	/home/max/Dropbox/4202/prog/test-dwarf/main.c:1
00000000 B g_my_private_global	/home/max/Dropbox/4202/prog/test-dwarf/main.c:3
00000000 T main	/home/max/Dropbox/4202/prog/test-dwarf/main.c:4
00000000 n wm4.0.ca8be4477412fcb0d732c8d55bace892
max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-gcc -g3 -gdwarf-4 -mthumb -mcpu=cortex-m4 -nostdlib -ffunction-sections -fdata-sections -O0 -fno-common example-O0.o -o example-O0.elf
/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../lib/gcc/arm-none-eabi/9.2.1/../../../../arm-none-eabi/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000008000
max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-nm -s -l example-O0.elf
0001802c B __bss_end__
0001802c B _bss_end__
00018024 B __bss_start
00018024 B __bss_start__
00018024 T __data_start
00018024 T _edata
0001802c B __end__
0001802c B _end
00018024 B g_my_externd_global
00018028 B g_my_private_global	/home/max/Dropbox/4202/prog/test-dwarf/main.c:3
00008000 T main	/home/max/Dropbox/4202/prog/test-dwarf/main.c:4
00080000 N _stack
         U _start

max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-gcc -g3 -gdwarf-4 -mthumb -mcpu=cortex-m4 -nostdlib -ffunction-sections -fdata-sections -O0 -fcommon -c main.c -o example-O0-fcommon.o 
max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-nm -s -l example-O0-fcommon.o 
00000004 C g_my_externd_global
00000004 C g_my_private_global
00000000 T main	/home/max/Dropbox/4202/prog/test-dwarf/main.c:4
00000000 n wm4.0.ca8be4477412fcb0d732c8d55bace892

https://i.postimg.cc/4XFPPnmK/Screenshot-2020-03-13-15-57-41.png

The differences do not seem to me to justify the different behaviors

best regards

Max

extern int g_my_externd_global;
int g_my_externd_global;
int g_my_private_global;
int main(void)
{
	g_my_externd_global=1;
	g_my_private_global=1;
	return 0;
}

max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-gcc -g3 -gdwarf-4 -mthumb -mcpu=cortex-m4 -nostdlib -ffunction-sections -fdata-sections -O1 -fno-common -c main.c -o example-O1.o 
max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-nm -s -l example-O1.o 
00000000 B g_my_externd_global
00000000 B g_my_private_global	/home/max/Dropbox/4202/prog/test-dwarf/main.c:3
00000000 T main	/home/max/Dropbox/4202/prog/test-dwarf/main.c:4
00000000 n wm4.0.771cfe8abdd6d09387c8f60d9b1eb4ff

max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-gcc -g3 -gdwarf-4 -mthumb -mcpu=cortex-m4 -nostdlib -ffunction-sections -fdata-sections -O0 -fno-common -c main.c -o example-O0.o 
max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-nm -s -l example-O0.o 
00000000 B g_my_externd_global	/home/max/Dropbox/4202/prog/test-dwarf/main.c:1
00000000 B g_my_private_global	/home/max/Dropbox/4202/prog/test-dwarf/main.c:3
00000000 T main	/home/max/Dropbox/4202/prog/test-dwarf/main.c:4
00000000 n wm4.0.ca8be4477412fcb0d732c8d55bace892
max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-gcc -g3 -gdwarf-4 -mthumb -mcpu=cortex-m4 -nostdlib -ffunction-sections -fdata-sections -O0 -fno-common example-O0.o -o example-O0.elf
/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../lib/gcc/arm-none-eabi/9.2.1/../../../../arm-none-eabi/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000008000
max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-nm -s -l example-O0.elf
0001802c B __bss_end__
0001802c B _bss_end__
00018024 B __bss_start
00018024 B __bss_start__
00018024 T __data_start
00018024 T _edata
0001802c B __end__
0001802c B _end
00018024 B g_my_externd_global
00018028 B g_my_private_global	/home/max/Dropbox/4202/prog/test-dwarf/main.c:3
00008000 T main	/home/max/Dropbox/4202/prog/test-dwarf/main.c:4
00080000 N _stack
         U _start

max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-gcc -g3 -gdwarf-4 -mthumb -mcpu=cortex-m4 -nostdlib -ffunction-sections -fdata-sections -O0 -fcommon -c main.c -o example-O0-fcommon.o 
max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-nm -s -l example-O0-fcommon.o 
00000004 C g_my_externd_global
00000004 C g_my_private_global
00000000 T main	/home/max/Dropbox/4202/prog/test-dwarf/main.c:4
00000000 n wm4.0.ca8be4477412fcb0d732c8d55bace892

https://i.postimg.cc/4XFPPnmK/Screenshot-2020-03-13-15-57-41.png

The differences do not seem to me to justify the different behaviors

best regards

Max

Maybe there's a way to instruct gcc to not split the records in the first place. (Edit: Unlikely, as it explicitly creates a new tuple for the definition, although this is done in order to support the info for C++ class-level static variables where the language forces the (non-const) static variable to be separately declared and defined).

Further investigation is needed for the fcommon-test. But it is likely, given the special nature of *COM* label (it isn't even a proper section in the .o file), that the lack of info can be easily explained away. You may want to debug binutils yourselves to find answer to that and other questions if any. It isn't necessary to build a cross-compiler in this case.

If the focus is on reviewing, reading and understanding software, cross-referencers (lxr, cscope, ctags, etags, or similar), and tools like Coccinelle, which can derive various relationships from the source-code, or even grep if the situation calls for it, are better options.

max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-gcc -g3 -gdwarf-4 -mthumb -mcpu=cortex-m4 -nostdlib -ffunction-sections -fdata-sections -O0 -fno-common -c main.c -o example-O0.o 
max@jarvis:~/Dropbox/4202/prog/test-dwarf$ arm-none-eabi-nm -s -l example-O0.o 
00000000 B g_my_externd_global	/home/max/Dropbox/4202/prog/test-dwarf/main.c:1
00000000 B g_my_private_global	/home/max/Dropbox/4202/prog/test-dwarf/main.c:3

Below is applicable to the O0-fno-common-test pasted above.

Given that the problem under consideration is about binutils' inability to print the expected info, one may be inclined to see all instances where it does print the expected info, as a sign that binutils is 'working as expected' at least in those instances.

That sign is not necessarily accurate.

Let P be the proposition "binutils doesn't print the expected info", and Q be the proposition "binutils is faulty".

Then, the proposition "P implies Q" can be generally accepted as true, assuming the expectations are valid (they are in this case).

But that says nothing about the truth-value of the proposition "~P implies ~Q".

The info for g_my_private_global was printed by binutils by its working as designed. It was able to fetch the information from the unbroken debug-record for that variable.

The info for g_my_externd_global happened to be printed, unintentionally, as if by a fluke. The phrase "even a stopped clock is right twice a day" explains binutils' behaviour here.

For simplicity, consider that the dwarf-debug-records maintain a (name, addr, file, line) tuple for each variable. The tool nm builds a query tuple, and tries to find a tuple in the dwarf-debug-records that match/satify the query.

But a variable, with its extern-decl-followed-by-definition in the same compilation-unit/source-file, has its debug-record split into two tuples.

For e.g., g_my_externd_global has two tuples instead of one:

• t0 = (name, AAA, file, line); points to the extern-decl.
• t1 = (BBB, addr, file, line); points to the definition.

Here, AAA denotes an invalid address; address isn't available at the extern-decl site.
BBB denotes an invalid name; name isn't collected from the definition site. (Note that a field (not shown here) in t1 links t1 to t0, so t1 indirectly has the name too).

binutils represents AAA by the value 0, and BBB by the value NULL. Therefore, the tuples for the g_my_externd_global variable are:

• t0 = ("g_my_externd_global", 0, file, line).
• t1 = (NULL, &g_my_externd_global, file, line).

When nm requests the info for g_my_externd_global, it passes a query-tuple q = ("g_my_externd_global", &g_my_externd_global).

Tuple t1 cannot satisfy q because t1.name != q.name. Any chance of getting the site/location of the definition thus evaporates because the definition-holding tuple is rejected.

If q.addr is 0, then t0 does satisfy q (although inadvertently).

With q.addr set to 0, t0 should ideally not satisfy q because the semantics of the two zero-address-values are different:

• In a query tuple, when addr is 0, it means that the variable's address is 0.
• In a dwarf-debug-record tuple, when addr is 0, it means that the variable's address isn't yet available in this tuple.

But, binutils does not seem to consider such semantic differences; it claims that t0 satifies q, and extracts/prints the (file, line) info from t0. All one gets then is the site of extern-decl.

When is &g_my_externd_global zero?

When nm generates the query, it calculates the address of the variable (in a couple of different ways).

Relevant for us is the equation, addr = section->vma + offset_var.

• section is the section inside which the variable resides.
• offset_var is the offset of the variable, relative to the containing-section's start.

If one dumps the section info for a .o file, it can be seen that section->vma is 0 for every section. Therefore, a variable that resides at offset 0 in its containing section (i.e. at the very start of the section) has an address = 0+0 = 0.

In case of g_my_externd_global, it so happens, perhaps because its definition appears first in the source, that it is the first variable to be placed in .bss (after resolving the effect of -fdata-sections on it). Thus, its address is calculated as 0.

To see the behaviour, one can run a few tests:

/* GT0 */

extern int g_my_externd_global;
extern int g_my_externd_global1;
extern int g_my_externd_global2;
extern int g_my_externd_global3;

/* Note below the order in which the symbols are defined. */
int g_my_externd_global;	/* Consider this line as slot#1 */
int g_my_externd_global1;
int g_my_externd_global2;
int g_my_externd_global3;
int g_my_private_global;

/* Result: (file,line) printed for g_my_externd_global and g_my_private_global.*/

Now swap g_my_externd_global with another externd global:

/* GT1 */
int g_my_externd_global3;
int g_my_externd_global1;
int g_my_externd_global2;
int g_my_externd_global;
int g_my_private_global;

/* Result: (file,line) printed for g_my_externd_global3 and g_my_private_global. */

Now swap g_my_externd_global3 with g_my_private_global:

/* GT2 */

int g_my_private_global;
int g_my_externd_global1;
int g_my_externd_global2;
int g_my_externd_global;
int g_my_externd_global3;

/* Result: (file,line) printed for g_my_private_global, but not for any of the externd variables. */

To have binutils print the tuples, find and modify the function lookup_symbol_in_variable_table inside binutils-2.34/bfd/dwarf2.c.

The function is of the form:

for
  if
    break;

for {
    printf("q = (%s,%lx), t = (%s, %lx)\n", name, addr, each->name, each->addr);
    if
        break;
}

(Re)build binutils, and run the newly-built nm/nm-new on the .o file. Sample output for the GT0 test below:

0000000000000000 B g_my_externd_global
q = (g_my_externd_global,0), t = (g_my_private_global, 10)
q = (g_my_externd_global,0), t = ((null), c)
q = (g_my_externd_global,0), t = ((null), 8)
q = (g_my_externd_global,0), t = ((null), 4)
q = (g_my_externd_global,0), t = ((null), 0) <----------------- Valid 0 address here, but name-mismatch.
q = (g_my_externd_global,0), t = (g_my_externd_global3, 0)
q = (g_my_externd_global,0), t = (g_my_externd_global2, 0)
q = (g_my_externd_global,0), t = (g_my_externd_global1, 0)
q = (g_my_externd_global,0), t = (g_my_externd_global, 0) <---- Match! But these two zeroes here aren't 'equal'.
        /home/user/extern/main.c:1 <--------------------------- Success!? Not really.

Edit: Added a comment about gcc in the conclusion above.

Edit2: gdb may be of some help:

[user@mach extern]$ gdb -q example-O0.elf
Reading symbols from example-O0.elf...
(gdb) info variables g_my_*
All variables matching regular expression "g_my_*":

File main.c:
5:	int g_my_externd_global;
6:	int g_my_externd_global1;
7:	int g_my_externd_global2;
8:	int g_my_externd_global3;
9:	int g_my_private_global;
(gdb)