Re: [PATCH] malloc: Correct the documentation of the top_pad default

[Siddhesh Poyarekar <siddhesh@gotplt.org>]

On 2022-08-04 04:48, Florian Weimer via Libc-alpha wrote:
> DEFAULT_TOP_PAD is defined as 131072 in
> sysdeps/generic/malloc-machine.h.

LGTM, but I wonder if we should tighten this so that it doesn't appear 
that DEFAULT_TOP_PAD could be 0 in any of our build configurations. 
e.g. we could replace the definition with a #error "define 
DEFAULT_TOP_PAD" or something like that to ensure that it's always 
defined through malloc-machine.h.  What do you think?

Thanks,
Sid

> 
> ---
>   elf/dl-tunables.list           | 1 +
>   elf/tst-rtld-list-tunables.exp | 2 +-
>   manual/tunables.texi           | 2 +-
>   3 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/elf/dl-tunables.list b/elf/dl-tunables.list
> index e6a56b3070..e925590c36 100644
> --- a/elf/dl-tunables.list
> +++ b/elf/dl-tunables.list
> @@ -42,6 +42,7 @@ glibc {
>         type: SIZE_T
>         env_alias: MALLOC_TOP_PAD_
>         security_level: SXID_IGNORE
> +      default: 131072
>       }
>       perturb {
>         type: INT_32
> diff --git a/elf/tst-rtld-list-tunables.exp b/elf/tst-rtld-list-tunables.exp
> index cdfdb56a94..2233ea9c7c 100644
> --- a/elf/tst-rtld-list-tunables.exp
> +++ b/elf/tst-rtld-list-tunables.exp
> @@ -9,7 +9,7 @@ glibc.malloc.perturb: 0 (min: 0, max: 255)
>   glibc.malloc.tcache_count: 0x0 (min: 0x0, max: 0x[f]+)
>   glibc.malloc.tcache_max: 0x0 (min: 0x0, max: 0x[f]+)
>   glibc.malloc.tcache_unsorted_limit: 0x0 (min: 0x0, max: 0x[f]+)
> -glibc.malloc.top_pad: 0x0 (min: 0x0, max: 0x[f]+)
> +glibc.malloc.top_pad: 0x20000 (min: 0x0, max: 0x[f]+)
>   glibc.malloc.trim_threshold: 0x0 (min: 0x0, max: 0x[f]+)
>   glibc.rtld.dynamic_sort: 2 (min: 1, max: 2)
>   glibc.rtld.nns: 0x4 (min: 0x1, max: 0x10)
> diff --git a/manual/tunables.texi b/manual/tunables.texi
> index 83cdcdac6d..ffd50a3fe2 100644
> --- a/manual/tunables.texi
> +++ b/manual/tunables.texi
> @@ -143,7 +143,7 @@ number of bytes to retain when shrinking any of the arenas.  This provides the
>   necessary hysteresis in heap size such that excessive amounts of system calls
>   can be avoided.
>   
> -The default value of this tunable is @samp{0}.
> +The default value of this tunable is @samp{131072} (128 KB).
>   @end deftp
>   
>   @deftp Tunable glibc.malloc.perturb
>