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Is the existence of LD_PRELOAD a strong argument in favor of static linking? I hadn't heard of LD_PRELOAD before now, but my first reaction was "oh wow, better

Is the existence of LD_PRELOAD a strong argument in favor of static linking? I hadn't heard of LD_PRELOAD before now, but my first reaction was "oh wow, better static link all the things!". Is that wrong?

dig1 7y ago
> "oh wow, better static link all the things!". Is that wrong? Depends. Sadly, current trend is to keep everything static in big binary or bundle own set of dependencies. Let's assume your program is using libpng. Now, every program is keeping own libpng in RAM. That is not even the worst case; imagine particular libpng has security issue. The only way to update all those static programs is to recompile everything or download everything; which is trend, again. I image we will have shared library approach "rediscovered" and commercialized with some cool buzzword.
- nicoburns 7y ago
  How much RAM does libpng use? It surely can't more than a couple of megabytes at most (much less I'd imagine, given typical binary sizes). This is rapidly becoming a non-issue. Regarding security issues. I imagine that we will gradually rewrite everything in memory-safe languages such as Rust. It'll take a while - there's a lot of value in the code that's already been written. But once a library for a given task has been rewritten into a safe language, it's going to be hard to justify using the equivalent C library. Of course this won't completely eliminate security issues. But I'd guess that along with a bit of fuzzing, it could easily reduce the frequency to <10% of what we have today. That, along with the fact that modern languages have package managers, and updating a library to the latest patch version is a one-line change, will make keeping on top of security issues well within the reach of the average app developer. Shared libraries were a necessary tool in a world with limited computing resources, but I suspect the current trend will continue until they're used only in niche circumstances (plugins? libc?)
  - zzzcpan 7y ago
    > How much RAM does libpng use? The assumption here is likely that read-only mapped pages of libpng could be shared across different running programs that link with it. But it ignores all the extra read-write pages as a consequence of dynamic linking and that there is basically just libc that is shared across different running programs. So it's mostly wrong, you can probably even reduce memory usage by switching to static linking.
    - cycloptic 7y ago
      Libpng is not a good example. Often in real-world applications there are dozens of libraries that are dynamically linked, and they can get quite large. The other argument is that having them dynamically linked increases cache coherency. For example if all applications share the same GUI library, the core rendering loop of that library will be likely to get kept in the cache. However, I have not tested this theory and it seems like the library would have to be designed for it to get any real benefit.
  - dig1 7y ago
    Libpng wasn't good example regarding size indeed, my main focus was security and how much of things it can affect. For heavy libraries, good example is Qt and KDE. KDE would preload all necessary libraries so KDE and Qt applications could start faster, and there was significant differences when Qt application was started under KDE comparing other desktop environment. Don't forget that under X11, everything is linked with X11 libraries and many applications with either Qt or Gtk, which brings own set of additional dependencies (pango, cairo, etc). > Regarding security issues. I imagine that we will gradually rewrite everything in memory-safe languages such as Rust I'm not sure how this will save us from programming errors. You just move from one surface (memory) to another (VM, compiler, package manager) with added complexity.
hoytech 7y ago
Why do you believe this? Are you under the impression there is some security problem, or are you opposed to users having more control over how programs run on their computers?
- codesections 7y ago
  The former. One of the examples given in the linked explanation (https://jvns.ca/blog/2014/11/27/ld-preload-is-super-fun-and-easy/ https://jvns.ca/blog/2014/11/27/ld-preload-is-super-fun-and-...) used LD_PRELOAD to modify random number generation — and it's easy to imagine other exploits. I suppose many/all would require the user's environment to be so throughly compromised that security might be a lost cause. But it still makes me wonder if there's a defense-in-depth argument in favor of static linking.
  - teddyh 7y ago
    A running program has no security boundary against the user the program is running as. This is not an “exploit”, this is as designed. A program is started by the user and running on behalf on the user – it should be 100% under the user’s control. If the program has additional privileges which should be withheld from the user, like original Unix setuid or setgid, or modern style Linux capabilities(7), then LD_PRELOAD is ignored by ld.so(8), and there is no problem. But if you are talking about a normal user’s environment being “compromised”, or the users’ wishes being a problem, then you have no business writing software for users, or, rather, users would be better off not running software written by you, since your software is obviously not written with their best interest in mind.
m45t3r 7y ago
For security reasons you say? No. I mean, LD_PRELOAD is very useful. I used libfaketime once to debug a problem in a project that I worked that only happened when you run the test suite in a specific range of time. This power is very useful. Can it be abused? Maybe, however you should assume that your environment is trusted or there is way worse ways to modify a process that is running in your system. In some of them it doesn't matter if the binary is static or not.
enriquto 7y ago
> Is the existence of LD_PRELOAD a strong argument in favor of static linking? There are so many strong arguments for static linking that this one feels like a drop in the swimming pool; but yes.

dig1 7y ago

> "oh wow, better static link all the things!". Is that wrong? Depends. Sadly, current trend is to keep everything static in big binary or bundle own set of dependencies. Let's assume your program is using libpng. Now, every program is keeping own libpng in RAM. That is not even the worst case; imagine particular libpng has security issue. The only way to update all those static programs is to recompile everything or download everything; which is trend, again. I image we will have shared library approach "rediscovered" and commercialized with some cool buzzword.

nicoburns 7y ago
How much RAM does libpng use? It surely can't more than a couple of megabytes at most (much less I'd imagine, given typical binary sizes). This is rapidly becoming a non-issue. Regarding security issues. I imagine that we will gradually rewrite everything in memory-safe languages such as Rust. It'll take a while - there's a lot of value in the code that's already been written. But once a library for a given task has been rewritten into a safe language, it's going to be hard to justify using the equivalent C library. Of course this won't completely eliminate security issues. But I'd guess that along with a bit of fuzzing, it could easily reduce the frequency to <10% of what we have today. That, along with the fact that modern languages have package managers, and updating a library to the latest patch version is a one-line change, will make keeping on top of security issues well within the reach of the average app developer. Shared libraries were a necessary tool in a world with limited computing resources, but I suspect the current trend will continue until they're used only in niche circumstances (plugins? libc?)
- zzzcpan 7y ago
  > How much RAM does libpng use? The assumption here is likely that read-only mapped pages of libpng could be shared across different running programs that link with it. But it ignores all the extra read-write pages as a consequence of dynamic linking and that there is basically just libc that is shared across different running programs. So it's mostly wrong, you can probably even reduce memory usage by switching to static linking.
  - cycloptic 7y ago
    Libpng is not a good example. Often in real-world applications there are dozens of libraries that are dynamically linked, and they can get quite large. The other argument is that having them dynamically linked increases cache coherency. For example if all applications share the same GUI library, the core rendering loop of that library will be likely to get kept in the cache. However, I have not tested this theory and it seems like the library would have to be designed for it to get any real benefit.
- dig1 7y ago
  Libpng wasn't good example regarding size indeed, my main focus was security and how much of things it can affect. For heavy libraries, good example is Qt and KDE. KDE would preload all necessary libraries so KDE and Qt applications could start faster, and there was significant differences when Qt application was started under KDE comparing other desktop environment. Don't forget that under X11, everything is linked with X11 libraries and many applications with either Qt or Gtk, which brings own set of additional dependencies (pango, cairo, etc). > Regarding security issues. I imagine that we will gradually rewrite everything in memory-safe languages such as Rust I'm not sure how this will save us from programming errors. You just move from one surface (memory) to another (VM, compiler, package manager) with added complexity.

hoytech 7y ago

Why do you believe this? Are you under the impression there is some security problem, or are you opposed to users having more control over how programs run on their computers?

codesections 7y ago
The former. One of the examples given in the linked explanation (https://jvns.ca/blog/2014/11/27/ld-preload-is-super-fun-and-easy/ https://jvns.ca/blog/2014/11/27/ld-preload-is-super-fun-and-...) used LD_PRELOAD to modify random number generation — and it's easy to imagine other exploits. I suppose many/all would require the user's environment to be so throughly compromised that security might be a lost cause. But it still makes me wonder if there's a defense-in-depth argument in favor of static linking.
- teddyh 7y ago
  A running program has no security boundary against the user the program is running as. This is not an “exploit”, this is as designed. A program is started by the user and running on behalf on the user – it should be 100% under the user’s control. If the program has additional privileges which should be withheld from the user, like original Unix setuid or setgid, or modern style Linux capabilities(7), then LD_PRELOAD is ignored by ld.so(8), and there is no problem. But if you are talking about a normal user’s environment being “compromised”, or the users’ wishes being a problem, then you have no business writing software for users, or, rather, users would be better off not running software written by you, since your software is obviously not written with their best interest in mind.

m45t3r 7y ago

For security reasons you say? No. I mean, LD_PRELOAD is very useful. I used libfaketime once to debug a problem in a project that I worked that only happened when you run the test suite in a specific range of time. This power is very useful. Can it be abused? Maybe, however you should assume that your environment is trusted or there is way worse ways to modify a process that is running in your system. In some of them it doesn't matter if the binary is static or not.

enriquto 7y ago

> Is the existence of LD_PRELOAD a strong argument in favor of static linking? There are so many strong arguments for static linking that this one feels like a drop in the swimming pool; but yes.