Support faster methods of reading process memory

[Jake-Shadle]

This adds 2 new methods for reading external process memory, in addition to the existing PTRACE_PEEKDATA approach currently used.

1. process_vm_readv - Reads contiguous blocks of a specified size, available since Linux 3.2, so realistically available in every reasonable environment...
2. /proc/{pid}/mem - As a fallback in case running in an ancient Linux, we can read from this procfs file, which also allows easy reading of blocks of memory.
3. PTRACE_PEEKDATA - The reliable but extremely slow fallback

These 3 methods are probed in the order above until one succeeds, as they all require the same permissions.

Resolves: #72

[gabrielesvelto]

I'm testing this in Firefox today, I want to get a feel of the speedup and make sure everything is working alright.

[Jake-Shadle]

Cool, I didn't actually do any benchmarking as this was just a side task while doing other stuff, so it'd be good to see in a real world scenario if this moves the needle (I would hope it would when doing 1 syscall vs n syscalls :p)

[gabrielesvelto]

Didn't have time to review properly but I did test it, and it's looking good! On my machine, using a debug build of Firefox it takes 160-180ms to grab a minidump w/o this patch. With this applied it drops to 50-75ms. I hope to have time to test an optimized build tomorrow. I also tried on a cheap Android tablet but the I/O is so slow there that the improvements are lost in the noise.

[Jake-Shadle]

I guess one thing to note is that I don't think the file path is being taken in any test so I'm not sure about that one, I'll add a test specifically for that before merging.

[gabrielesvelto]

Sorry for the super-long delay here but I've finally had time to come back to this. I've tested this extensively on Linux builds and I see at minimum a speedup of an order of magnitude. When running Firefox crash generation tests in debug builds the speedup is 25 times (you read that right, ~12ms with the patch applied vs ~300ms without it). This is going to be extremely useful.

I didn't have time to test Android again but I expect larger gains there (because syscalls are slower) but maybe a smaller impact in the overall time because I/O is also slower and takes a larger portion of the time.

[gabrielesvelto]

@Jake-Shadle if you don't have other changes to make we can merge it today.

[Jake-Shadle]

Ahh sounds good, I just want to add a test specifically for the file case since that will never currently be hit on CI/my local machine, then we can merge.