[U-Boot] [PATCH] Prevent malloc with size 0

[Wolfgang Denk]

Dear Marek Vasut,

In message <201210160831.20759.marex at denx.de> you wrote:
> 
> > In short, returning non-NULL from malloc(0) and expecting a CPU exception
> > when it is de-referenced is not going to fly.

We should not expect to have support for any exceptions for any kind
of illegal accesses.  In general, behaviours is undetermined.

> > [1] Apparently the way do do it is to reserve the entire first 4kB page and
> > mark it as 'not-present' so any access causes a page-fault.
> 
> Ok, I don't mean to reopen this can of worms again ... but what're we going to 
> do about this patch?

NAK it.

It is perfectly valid on most systems to dereference a pointer to
address 0 (which in almost all cases looks the same as a NULL
pointer).

Test on ARM (some i.MX31 board):

=> md 0 20
00000000: e59ff00c e59ff018 e59ff018 e59ff018    ................
00000010: e59ff018 a0000000 e59ff014 e59ff014    ................
00000020: 00000090 1fffffd0 1fffffd4 1fffffd8    ................
00000030: 1fffffdc 1fffffe0 1fffffe4 ffffffff    ................
00000040: 79706f43 68676972 63282074 30322029    Copyright (c) 20
00000050: 4d203430 726f746f 20616c6f 2e636e49    04 Motorola Inc.
00000060: 6c6c4120 67697220 20737468 65736572     All rights rese
00000070: 64657672 0000002e ffffffff ffffffff    rved............

Test on PPC (some MPC5200 board):

=> md 0 10
00000000: 60000000 60000000 60000000 2c050000    `...`...`...,...
00000010: 4182001c 429f0005 7d0802a6 3d080000    A...B...}...=...
00000020: 3908ffe8 483a40c1 7fe00008 7c7f1b78    9...H:@.....|..x
00000030: 3b000000 483a2739 48003519 48003465    ;...H:'9H.5.H.4e


I object against patches that will make access to this data impossible
(or even more complicated than it is now).

Best regards,

Wolfgang Denk

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