Re: [PATCH] gdb/testsuite: Allow for failure to read some memory addresses

[Simon Marchi <simark at simark dot ca>]

On 2018-07-04 02:13 PM, Andrew Burgess wrote:
> In the gdb.base/examine-backward.exp test script, we check to see if
> address zero is readable, and then read memory first forward from
> address zero, and then backward from address zero.
> 
> The problem is, being able to read address zero does not guarantee
> that you'll be able to read from the other end of the address space,
> and the test probably shouldn't assume that is the case.
> 
> This patch extends the success conditions so that, even if GDB fails
> to read memory, so long as the error message indicates that GDB was
> trying to access the correct location, then we consider this a pass.
> The test is, I think, trying to show that GDB can compute the correct
> address when going backward from zero, being able to access the memory
> at that address is secondary.
> 
> One further change is that, when we examined the memory at address
> zero, the regexp used to match the address expected that the zero
> address would have two '0' digits as the least significant digits.  As
> GDB strips leading zeros from addresses this was causing the test to
> fail.  I've reduced the zero address to a single 0 digit.

Hi Andrew,

This probably means that we could run the test even if address 0 is not readable?
On x86_64, I get:

(gdb) x/3b 0
0x0:	Cannot access memory at address 0x0
(gdb) x/-6b
0xfffffffffffffffb:	Cannot access memory at address 0xfffffffffffffffb

It's not the exact same result as if address 0 is readable (since we didn't
get to read all three bytes in the first command), but it still reaches
the goal of the test.

> +	set test "examine 6 bytes backward"
> +	gdb_test_multiple "x/-6x" "$test" {
> +	    -re "0x\[0-9a-f\]+fd.*:${byte}${byte}${byte}${byte}${byte}${byte}.*\[\r\n\]*$gdb_prompt $" {
> +		pass $test
> +	    }
> +	    -re "0x\[0-9a-f\]+fd.*:\tCannot access memory at address 0x\[0-9a-f\]+fd.*\[\r\n\]*$gdb_prompt $" {
> +		# We test that we can read zero, but that's no
> +		# guarantee that we can read from the other end of the
> +		# address space.  If we get an error about trying to
> +		# read from the expected address then we count that as
> +		# a pass, GDB did try to read the correct location.
> +		pass $test
> +	    }
> +	    -re "$gdb_prompt $" {
> +		fail $test
> +	    }

Is the last stanza necessary?  From what I remember, if it doesn't match
any other one, it will already fail the test.

Simon