[lldb-dev] how to set a watchpoint on an "unsigned short" parameter ?
Ted Woodward via lldb-dev
lldb-dev at lists.llvm.org
Fri Feb 15 08:46:59 PST 2019
I don't see anything on this line that would change x:
fprintf(stderr, "some string %lu %c\n",
r==(void*)0UL)? 255UL : A_MACRO(r),
x? '0' : '1'
);
I suggest you set a breakpoint on the line and a breakpoint on the next line. Verify that x is the wrong value using "frame variable x" when you hit the second breakpoint.
Make sure this breakpoint is on a source line that does something, not in the middle of the function return.
If the value changes, disassemble the line and set a breakpoint on the call instruction for printf. See if the value changes when you hit that breakpoint.
If it does, look at the assembly and see if anything is changing the data at the address of x. You can get the address with "frame variable &x".
If the call to printf is changing the value of x, you've probably got stack corruption going on.
-------------------------------------------------------------------------------------------
Clang is supported on Linux:
>uname
Linux
>which clang
/usr/bin/clang
So you aren't forced to use gcc just because you're targeting Linux.
-------------------------------------------------------------------------------------------
watchpoint set syntax is as follows:
(lldb) help watch set
Commands for setting a watchpoint.
Syntax: watchpoint set <subcommand> [<subcommand-options>]
If you're setting it on a variable and you have good DWARF info, you don't need to specify a size. From my example:
> (lldb) w s v i
> Watchpoint created: Watchpoint 1: addr = 0x0410eec6 size = 2 state =
> enabled type = w
-----Original Message-----
From: Jason Vas Dias <jason.vas.dias at gmail.com>
Sent: Friday, February 15, 2019 10:33 AM
To: Ted Woodward <tedwood at quicinc.com>
Cc: LLDB <lldb-dev at lists.llvm.org>
Subject: [EXT] Re: [lldb-dev] how to set a watchpoint on an "unsigned short" parameter ?
Good day Ted -
Thanks for responding - but I did try that one:
(lldb) wa s v x
error: Watchpoint creation failed (addr=0xffffffffffffffff, size=0, variable
expression='x').
error: cannot set a watchpoint with watch_size of 0
(lldb) wa s -s 2 v x
invalid command 'watchpoint set -s'.
(lldb) wa s v -s 2 x
error: Watchpoint creation failed (addr=0xffffffffffffffff, size=0, variable
expression='x').
error: cannot set a watchpoint with watch_size of 0
I can't seem to get lldb to recognize the '-s' / '--size' options no matter
where I put them . And the documentation, such as it is,
(on https://lldb.llvm.org/lldb-gdb.html), is very vague and incomplete.
I guess my problems are because I am compiling with GCC , and trying
to debug with LLDB .
But since the program I am debugging is targetted mainly for the Linux platform,
(I am just using MacOSX for testing) I wanted to compile with GCC .
I guess it is not possible to debug GCC compiled programs with LLDB ?
The problem I am trying to track down is stack corruption caused by
fprintf() :
void f ( void *r, unsigned short x )
{ ...
fprintf(stderr, "some string %lu %c\n",
r==(void*)0UL)? 255UL : A_MACRO(r),
x? '0' : '1'
);
// after this fprintf, the value of x changes from 12 to 8630 .
// why ? It would be nice to be able to use LLDB to find out,
// but this does not work.
}
I've just had to comment out the fprintf , so the problem does not occur.
Coming from a background of using GDB for the past 25 years, I find
this lack of watchpoint support in LLDB very difficult to accept.
Thanks & Best Regards,
Jason
On 15/02/2019, Ted Woodward <tedwood at quicinc.com> wrote:
> "w s v x" would be the command you want.
>
>
> (lldb) b f
> Breakpoint 1: where = watch`f + 12 at watch.c:5:4, address =
> 0x000050ec
> (lldb) r
> hexagon-sim INFO: The rev_id used in the simulation is 0x00004060
> (v60a_512)
> hexagon-sim INFO: Setting up debug server on port 57824 Process 1
> launched: '/usr2/tedwood/lldb_test/watch' (hexagon) Process 1 stopped
> * thread #1, name = 'T1', stop reason = breakpoint 1.1
> frame #0: 0x000050ec watch`f(i=2) at watch.c:5:4
> 2
> 3 unsigned short f(unsigned short i)
> 4 {
> -> 5 i++;
> 6 return i;
> 7 }
> 8
> (lldb) w s v i
> Watchpoint created: Watchpoint 1: addr = 0x0410eec6 size = 2 state =
> enabled type = w
> declare @ '/usr2/tedwood/lldb_test/watch.c:3'
> watchpoint spec = 'i'
> new value: 2
> (lldb) c
> Process 1 resuming
>
> Watchpoint 1 hit:
> old value: 2
> new value: 3
> Process 1 stopped
> * thread #1, name = 'T1', stop reason = watchpoint 1
> frame #0: 0x000050f8 watch`f(i=3) at watch.c:6:10
> 3 unsigned short f(unsigned short i)
> 4 {
> 5 i++;
> -> 6 return i;
> 7 }
> 8
> 9 int main(int argc, char **argv)
>
>
>
> -----Original Message-----
> From: lldb-dev <lldb-dev-bounces at lists.llvm.org> On Behalf Of Jason
> Vas Dias via lldb-dev
> Sent: Thursday, February 14, 2019 1:28 PM
> To: lldb-dev at lists.llvm.org
> Subject: [lldb-dev] how to set a watchpoint on an "unsigned short"
> parameter ?
>
> Good day -
>
> I'd be most grateful if anyone could enlighten me as to how
> to set a watchpoint on an unsigned short parameter variable
> in lldb .
>
> I am trying to follow the instructions at :
> https://lldb.llvm.org/lldb-gdb.html
> but they do not work to set watchpoints.
>
> There seems to be no other documentation for LLDB commands -
> or if anyone knows of any , please let me know.
>
> I have a function like :
> void f ( unsigned short x )
> { .... }
>
> With the debugger stopped inside f, I have tried:
>
> (lldb) p &x
> (uint16_t *) $3 = 0x00000001001122c0
> (lldb) wa s v -s 2 -w write 0x00000001001122c0
> error: no variable named '0x00000001001122c0' found in this frame
> (lldb) wa s v -s 2 -w write x
> error: Watchpoint creation failed (addr=0xffffffffffffffff, size=0,
> variable
> expression='x').
> error: cannot set a watchpoint with watch_size of 0
> (lldb) wa s e -s 2 -w write 0x00000001001122c0
> error: expression evaluation of address to watch failed
> expression evaluated: -s 2 -w write 0x00000001001122c0
> (lldb) wa s e -s 2 -w write *0x00000001001122c0
> error: expression evaluation of address to watch failed
> expression evaluated: -s 2 -w write *0x00000001001122c0
> (lldb) wa s e -s 2 -w write ((unsigned short*)0x00000001001122c0)
> error: expression evaluation of address to watch failed
> expression evaluated: -s 2 -w write ((unsigned
> short*)0x00000001001122c0)
> (lldb) wa s v -s 2 -w write &x
> error: 'x' doesn't have a valid address
> # ^- this error is really strange, particularly as I can do:
> (lldb) p &x
> (uint16_t *) $5 = 0x00000001001122c0
>
> It seems to me lldb's implementation of watch points is
> fundamentally broken -
> there is no way I've been able to get it to work .
>
> Unfortunately, I have to use MacOSX, so gdb is not available.
>
> Please, can anyone suggest how to successfully set a watchpoint on
> a parameter (stack) located variable value with lldb ?
> It does not seem to be possible.
>
> My next step, if no answers to this mail, would be to analyse the LLDB
> source code to see if I can figure out how watchpoints are meant to
> be set, seeing as there is no reference documentation for LLDB commands,
> either installed as manual pages or online. This to me makes LLDB
> unsuitable
> for production use, but unforunately I have to use it (I need to
> debug under
> MacOSX 10,14.3 ).
>
> The help output for is of no use either:
> (lldb) help watch set
> "Syntax: watchpoint set <subcommand> [<subcommand-options>]
> The following subcommands are supported:
> expression -- Set a watchpoint on an address by supplying an
> expression. Use the
> '-w' option to specify the type of watchpoint and the '-s'
> option to specify the
> byte size to watch for.
> "
> The above statement is provably false:
> (lldb) wa s v -s 2 x
> error: Watchpoint creation failed (addr=0xffffffffffffffff, size=0,
> variable expression='x').
> error: cannot set a watchpoint with watch_size of 0
> # maybe the -s option goes after the 'set' ? no:
> (lldb) wa s -s 2 v reader_id
> invalid command 'watchpoint set -s'.
>
> All attempts to
> "Use the '-w' option to specify the type of watchpoint and the '-s'
> option to
> specify the byte size to watch for.
> "
> fail, so there must be alot missing from the help description.
>
> The help for the variable syntax is also vague, and provably false :
> "variable -- Set a watchpoint on a variable. Use the '-w'
> option to specify the type
> of watchpoint and the '-s' option to specify the
> byte size to watch for.
> If no '-w' option is specified, it defaults to
> write. If no '-s' option is
> specified, it defaults to the variable's byte
> size. Note that there are
> limited hardware resources for watchpoints. If
> watchpoint setting
> fails, consider disable/delete existing ones to
> free up resources.
>
> (lldb) wa s v x
> error: Watchpoint creation failed (addr=0xffffffffffffffff,
> size=0, variable
> expression='x').
> error: cannot set a watchpoint with watch_size of 0
> (lldb) wa s -s 2 v x
> invalid command 'watchpoint set -s'.
> (lldb) wa s v -s 2 x
> error: Watchpoint creation failed (addr=0xffffffffffffffff,
> size=0, variable
> expression='x').
> error: cannot set a watchpoint with watch_size of 0
>
> So lldb's watch command can neither get the size from a '-s'
> option, nor can
> it get the size from sizeof(variable) . How then is one meant to
> set the size ?
>
> Obviously, lldb's watchpoint implementation needs much further work
> - has
> anyone been able to get it to do anything useful ? If so, please
> let me know how,
> and what.
>
> If there is any reference documentation for lldb, besides the false
> and misleading
> 'https://lldb.llvm.org/lldb-gdb.html', which actually describes in
> detail and
> correctly the syntax of every lldb command, please let me know.
>
> Any ideas gratefully received.
>
> Thank You & Best Regards,
> Jason Vas Dias
> _______________________________________________
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> lldb-dev at lists.llvm.org
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