RE: [PATCH v2] sysinfo: include availram field in sysinfo struct

From: Pintu Agarwal

Sent: 08 January 2022 16:53

On Sat, 8 Jan 2022 at 03:52, David Laight [off-list ref] wrote:

quoted

From: Pintu Kumar

quoted

Sent: 07 January 2022 18:08

The sysinfo member does not have any "available ram" field and
the bufferram field is not much helpful either, to get a rough
estimate of available ram needed for allocation.

One needs to parse MemAvailable field separately from /proc/meminfo
to get this info instead of directly getting if from sysinfo itself.

Thus, this patch introduce a new field as availram in sysinfo
so that all the info total/free/available can be retrieved from
one place itself.

...

quoted

diff --git a/include/uapi/linux/sysinfo.h b/include/uapi/linux/sysinfo.h
index 435d5c2..fe84c6a 100644
--- a/include/uapi/linux/sysinfo.h
+++ b/include/uapi/linux/sysinfo.h

@@ -19,7 +19,8 @@ struct sysinfo {
      __kernel_ulong_t totalhigh;     /* Total high memory size */
      __kernel_ulong_t freehigh;      /* Available high memory size */
      __u32 mem_unit;                 /* Memory unit size in bytes */
-     char _f[20-2*sizeof(__kernel_ulong_t)-sizeof(__u32)];   /* Padding: libc5 uses this.. */

There are 4 pad bytes here on most 64bit architectures.

quoted

+     __kernel_ulong_t availram;      /* Memory available for allocation */
+     char _f[20-3*sizeof(__kernel_ulong_t)-sizeof(__u32)];   /* Padding: libc5 uses this.. */
 };

You've not compile-time tested the size of the structure.

With "32" instead of "20" in padding I get these size of sysinfo:
In x86-64 kernel, with app 64-bit: Size of sysinfo = 128
In x86-64 kernel, with app 32-bit:: Size of sysinfo = 76
In arm-64 kernel, with app 32-bit: Size of sysinfo = 76

You need to compare the sizes before and after your patch
to ensure it doesn't change on any architecture.

Okay the sys robot reported some issue in 64-bit build.
{{{

quoted

quoted

include/uapi/linux/sysinfo.h:23:14: error: size of array '_f' is too large
   23 |         char _f[20-3*sizeof(__kernel_ulong_t)-sizeof(__u32)];   /* Padding: libc5 uses

this.. */

quoted

quoted

      |              ^~

}}}

Also, I got the same issue while building for arm64, so I tried to
adjust like this:
char _f[32-3*sizeof(__kernel_ulong_t)-sizeof(__u32)];

With this the build works on both 32/64 but output fails when running
32-bit program on 64-bit kernel.
Also, the free command on 64-bit reports "stack smashing error"..

How do we resolve this issue to make it work on both arch ?
Also, I don't really understand the significance of that number "20"
in padding ?

My guess is that someone added a char _f[20] pad to allow for expansion.
Then two __kernel_ulong_t and one __u32 field were added, so the
size of the pad was reduced.
When __kernel_ulong_t is 64bit then it seems to be char _f[0]
- which might generate a compile warning since you are supposed
to use char _f[] to indicate an extensible structure.
There is, however, 4 bytes of pad after the _f[] on most 64bit
architectures.

So actually there isn't enough space to anything useful at all.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)