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Shell Programming and Scripting
Safely parsing parameters
Post 302406601 by Corona688 on Tuesday 23rd of March 2010 11:18:01 AM
03-23-2010
Sorry, I didn't notice this reply.
They are strings being fed into the kernel commandline itself, and being processed by my initramfs system by a full-fledged BASH shell. It occurred to me that splitting at the shell level like this was both very powerful and perilous, so I wondered if there was a general solution to this whole class of problems.
The perl solution looks very nice. It wouldn't be hard to feed it backticks instead of processing them first the way I get the data from the kernel. Unfortunately perl is a bit weighty to cram into an initramfs bootstrap loader.
But on second thought -- doesn't perl have backticks too?
I don't think my original post was "unbelievably vague". The problem is the same no matter what the ultimate purpose -- splitting arguments intelligently in a shell without permitting any expansions or substitutions. Whether or not the code is executing with elevated permissions, this isn't the sort of thing you want to allow just incidentally.
To process and evaluate the commands I must first divide them so I know what it would actually be doing, otherwise I'm just doing ad-hoc "injection rejection". I could write my own char-by-char shell parser inside the shell I suppose but this seems overkill. I could also make an escape-everything regex to make the string safe before eval-ing it but it's hard to prove there's absolutely no holes or omissions in a system like that. Or I could just strip out all dollar signs and backticks, but what if someday I need to pass a literal backtick for some reason?
I was hoping there was some obvious and more elegant way I was missing I suppose. Oh well, thanks for your responses.
They are strings being fed into the kernel commandline itself, and being processed by my initramfs system by a full-fledged BASH shell. It occurred to me that splitting at the shell level like this was both very powerful and perilous, so I wondered if there was a general solution to this whole class of problems.
The perl solution looks very nice. It wouldn't be hard to feed it backticks instead of processing them first the way I get the data from the kernel. Unfortunately perl is a bit weighty to cram into an initramfs bootstrap loader.
But on second thought -- doesn't perl have backticks too?I don't think my original post was "unbelievably vague". The problem is the same no matter what the ultimate purpose -- splitting arguments intelligently in a shell without permitting any expansions or substitutions. Whether or not the code is executing with elevated permissions, this isn't the sort of thing you want to allow just incidentally.
To process and evaluate the commands I must first divide them so I know what it would actually be doing, otherwise I'm just doing ad-hoc "injection rejection". I could write my own char-by-char shell parser inside the shell I suppose but this seems overkill. I could also make an escape-everything regex to make the string safe before eval-ing it but it's hard to prove there's absolutely no holes or omissions in a system like that. Or I could just strip out all dollar signs and backticks, but what if someday I need to pass a literal backtick for some reason?
I was hoping there was some obvious and more elegant way I was missing I suppose. Oh well, thanks for your responses.
Last edited by Corona688; 03-23-2010 at 12:37 PM..
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LEARN ABOUT OSX
platform::shell
platform::shell(n) Tcl Bundled Packages platform::shell(n) __________________________________________________________________________________________________________________________________________________ NAME
platform::shell - System identification support code and utilities SYNOPSIS
package require platform::shell ?1.1.4? platform::shell::generic shell platform::shell::identify shell platform::shell::platform shell _________________________________________________________________ DESCRIPTION
The platform::shell package provides several utility commands useful for the identification of the architecture of a specific Tcl shell. This package allows the identification of the architecture of a specific Tcl shell different from the shell running the package. The only requirement is that the other shell (identified by its path), is actually executable on the current machine. While for most platform this means that the architecture of the interrogated shell is identical to the architecture of the running shell this is not generally true. A counter example are all platforms which have 32 and 64 bit variants and where a 64bit system is able to run 32bit code. For these running and interrogated shell may have different 32/64 bit settings and thus different identifiers. For applications like a code repository it is important to identify the architecture of the shell which will actually run the installed packages, versus the architecture of the shell running the repository software. COMMANDS
platform::shell::identify shell This command does the same identification as platform::identify, for the specified Tcl shell, in contrast to the running shell. platform::shell::generic shell This command does the same identification as platform::generic, for the specified Tcl shell, in contrast to the running shell. platform::shell::platform shell This command returns the contents of tcl_platform(platform) for the specified Tcl shell. KEYWORDS
operating system, cpu architecture, platform, architecture platform::shell 1.1.4 platform::shell(n)