/proc is not readably by readFile

[guibou]

readFile and similar operations disallows reading from /proc. For example, on my system, /proc/meminfo contains a few lines, such as:

MemTotal:        8067164 kB
MemFree:         2833268 kB
...

However, reading them using builtins.readFile or "${/proc/meminfo}" results in empty string, without any warning / error:

$ nix repl
Welcome to Nix version 2.3.4. Type :? for help.

nix-repl> builtins.readFile "/proc/meminfo"
""

or:

nix-repl> f = "${/proc/meminfo}"
nix-repl> f                      
"/nix/store/rjxx1xy628gqs1nbl931xjahh5zr3cax-meminfo"
...

$ cat /nix/store/rjxx1xy628gqs1nbl931xjahh5zr3cax-meminfo

I don't understand this limitation.

I do understand that /proc output are not reproducibles, but this is the case of any file directly picked on the file system. I can use readFile on arbitrary files, such as builtins.readFile "/home/guibou/.vimrc" without any limitation, so why an arbitrary one on /proc`?

Actually, there is a trivial non-hermetic and dangerous workaround, /proc is readable at build time:

nix-repl> :l <nixpkgs>
Added 11766 variables.

nix-repl> :b runCommand "foo" {} ''cat /proc/meminfo > $out''                 
[1 built, 0.0 MiB DL]

this derivation produced the following outputs:
  out -> /nix/store/qfv5xyrxpll38y8iklymcw48y2sks985-foo

...

$ cat /nix/store/qfv5xyrxpll38y8iklymcw48y2sks985-foo | head -n 3
MemTotal:        8067164 kB
MemFree:         2829544 kB
MemAvailable:    4752268 kB

And that's normal, programs in the build sandbox must be able to read /proc to be able to work correctly. But this is really dangerous because this derivation is non reproducible at all. I can fix this problem by adding a builtins.currentTime in my derivation.

My feature request / discussion contains a few points:

• I'd like to understand the limitation on /proc
• Either remove this limitation or fail with an error or warning instead of an empty string.

[PkmX]

I think this is just due to the way Nix reads files:

nix/src/libutil/util.cc

Line 311 in c9d0cf7

if (fstat(fd, &st) == -1)

that it calls fstat to retrieve the file size so it can pre-allocate the buffer, but many files under /proc or /sys or /dev just report 0 or outright incorrect size.

[matthewbauer]

I think this is a dupe of #3026

[guibou]

@matthewbauer Indeed, thank you. I can close that one and continue the discussion on 3026.