Dear Marek Vasut,
In message 201210160831.20759.marex@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