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a16b03



Joined: 24 Jan 2006
Posts: 43
Location: Riga, Latvia
a16b03
When you call kernel to (for example) open file, kernel returns file descriptor or error.

I would like to know what is smallest possible value of file descriptor returned by kernel.

The idea is to compare smallest value (or largest value of error code) to returned value in eax so we won't need to check every possible error if not necessary.

Hope you get the point.
Post 07 Dec 2006, 22:42
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vid
Verbosity in development


Joined: 05 Sep 2003
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vid
linux kernel error codes are in range -4096 to 0. so you can do:
Code:
cmp eax, -4096
ja error    
Post 07 Dec 2006, 22:49
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a16b03



Joined: 24 Jan 2006
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a16b03
vid wrote:
linux kernel error codes are in range -4096 to 0.


Are you shore?
Post 07 Dec 2006, 23:08
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gunblade



Joined: 19 Feb 2004
Posts: 209
gunblade
Well, not 0, 0 is usually used to mean "all went well", but it is -4096 to -1 to indicate error as far as i know. Each one means a different error (although most syscalls probably wont implement all the error codes, just any which are relevant to what they are doing).

As for the lowest file descriptor which a syscall will return, it should technically be 0, if there is a syscall to return the stdin file descriptor.

FD Description
0 Stdin
1 Stdout
2 Stderr
3+ Used for files, pipes, threads.. anything which requires a file descriptor.
Post 08 Dec 2006, 18:53
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f0dder



Joined: 19 Feb 2004
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f0dder
Aren't you just supposed to check for errors by "num < 0"? It'd be more work checking actual ranges then, and you could end up in trouble that way...
Post 08 Dec 2006, 22:57
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
f0dder, are you talking about vid's code? It works, I'm still not convincing myself of how it works but after a few test it worked. Thanks for post it vid Very Happy
Post 09 Dec 2006, 00:28
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gunblade



Joined: 19 Feb 2004
Posts: 209
gunblade
f0dder, checking for < 0 will check between 0x8000000 and 0xFFFFFFFF (for 32-bits), which is much too large of a range, values returned are valid from 0 up to 0xFFFFEFFF, then between 0xFFFFF000 and 0xFFFFFFFF are the possible error values.
Post 13 Dec 2006, 21:45
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f0dder



Joined: 19 Feb 2004
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f0dder
gunblade wrote:
f0dder, checking for < 0 will check between 0x8000000 and 0xFFFFFFFF (for 32-bits), which is much too large of a range, values returned are valid from 0 up to 0xFFFFEFFF, then between 0xFFFFF000 and 0xFFFFFFFF are the possible error values.


Are they, now?

If the code internally uses signed ints to represent errors, then I very much doubt this.

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Post 13 Dec 2006, 23:00
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vid
Verbosity in development


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vid
f0dder: this is following standards, not particular implementation of those standards. And standards AFAIK say that error code is -4096 to -1, which CAN be tested by one cmp
Post 13 Dec 2006, 23:20
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f0dder



Joined: 19 Feb 2004
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f0dder
vid: okay - got a link to manpage, source file, or similar? Smile

Just keep in mind that most C code probably checks for "<0", so...
Post 13 Dec 2006, 23:55
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vid
Verbosity in development


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vid
f0dder: got logic.

In linux you have 3GB user-mode address space. What if mmap() returns you pointer above 2GB?

Also, from what i have read, even memory page xFFFFE000 to 0xFFFFEFFF is mapped. Then it seems logical to choose those values as error codes, which cannot act as pointer (those which immediately throw GPF)

Razz Wink
Post 14 Dec 2006, 03:51
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vid
Verbosity in development


Joined: 05 Sep 2003
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vid
f0dder: after some exploring... i realized it's probably not a standard. But still it is the logical range. <0 just wouldn't work well. And keep in mind "c code" is using libc, not linux kernel.
Post 14 Dec 2006, 12:40
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f0dder



Joined: 19 Feb 2004
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f0dder
I thought this was about file descriptors - but I see it has been expanded to a more generic "kernel error returns" Smile

Well, I've never bothered doing syscalls directly on *u*x (and I can't be bothered), so I wouldn't know if there's 100% generic rules... I'd be surprised if there were.
Post 14 Dec 2006, 15:08
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arafel



Joined: 29 Aug 2006
Posts: 131
Location: Jerusalem, Israel
arafel
MAX_ERRNO is 4095, hence the error values are in the range -4095 .. -1.
Simply checking the sign (since 'test eax, eax' is smaller and faster than 'cmp eax, -4096') will work fine for almost all system calls.
Post 14 Dec 2006, 15:38
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vid
Verbosity in development


Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia
vid
Quote:
Simply checking the sign (since 'test eax, eax' is smaller and faster than 'cmp eax, -4096') will work fine for almost all system calls.
but unfortunately, not for all. And using non-generic method you risk you will get used to it, and use it even when you shouldn't (like mmap).
Post 14 Dec 2006, 17:27
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