Re: [PATCH] Fix crash when loading a core with unexpected register section size

[Pedro Alves <palves at redhat dot com>]

On 01/26/2017 01:17 PM, Antoine Tremblay wrote:
> When loading a core without an executable like so:
> gdb --core core for example often the gdbarch won't contain the
> iterate_over_regset_sections method.

Can you give an example?  That'd help a lot understand the issue
better.

Also, please add a line break, ""s and/or punctuation to make
the command stand out more from the rest of the sentence.
For example:

When loading a core without an executable like so:
 $ gdb --core core
for example, often the gdbarch won't contain the
iterate_over_regset_sections method.  For example arch-foo.

> 
> This will generate a call to get_core_register_section with a NULL regset
> like at corelow.c:628
> 
> get_core_register_section (regcache, NULL, ".reg", 0, 0, "general-purpose", 1);
> 
> However a check for REGSET_VARIABLE_SIZE in get_core_register_section
> assumes that regset is != NULL thus leading to a crash with this backtrace:
> 
> (gdb) bt
> #0  0x000000000065907b in get_core_register_section
>     (regcache=regcache@entry=0x2c26260, regset=regset@entry=0x0,
>     name=name@entry=0xdbf7b2 ".reg", min_size=min_size@entry=0,
>     which=which@entry=0, human_name=human_name@entry=0xdbac28
>     "general-purpose", required=1)
>     at ../../gdb/corelow.c:542
> #1  0x0000000000659b70 in get_core_registers (ops=<optimized out>,
>     regcache=0x2c26260, regno=<optimized out>) at ../../gdb/corelow.c:628
> #2  0x000000000076e5fb in target_fetch_registers
>     (regcache=regcache@entry=0x2c26260, regno=regno@entry=15) at ../../gdb/target.c:3590
> 
> Note that commit: f962539ad23759af4ba8f7eece1946fdc2f5087 

Please always paste the commit's subject as well, to make
it easier for us poor humans to quickly tell what the commit
was about without having to go to a terminal.  The Linux guideline
is to put it in parens:

 Note that commit f962539ad23759 ("Warn if core file register
 section is larger than expected") introduced [...]

I personally like that style.

introcuded this

(typo)

> issue. Thus releases > 7.8.2 are affected.
> 
> This patch fixes this crash by adding a check for regset existence before
> running the condition.
> 
> gdb/ChangeLog:
> 
> 	* corelow.c (get_core_register_section): Check for regset
>                    existance before checking for REGSET_VARIABLE_SIZE.

Indentation.  "existence".

> ---
>  gdb/corelow.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/gdb/corelow.c b/gdb/corelow.c
> index a075d9e..f43f730 100644
> --- a/gdb/corelow.c
> +++ b/gdb/corelow.c
> @@ -515,6 +515,7 @@ get_core_register_section (struct regcache *regcache,
>    struct bfd_section *section;
>    bfd_size_type size;
>    char *contents;
> +  bool variable_size_section = false;

No need to initialize by default when you're always going to
initialize it again below.  Or declare on first use and avoid
the issue entirely.

Thanks,
Pedro Alves