Em Wed, Mar 01, 2023 at 09:07:14AM +1100, Dave Chinner escreveu: > On Tue, Feb 28, 2023 at 10:31:57AM +0100, Jiri Olsa wrote: > > this is RFC patchset for adding build id under inode's object. > > The main change to previous post [1] is to use inode object instead of file > > object for build id data. > > Please explain what a "build id" is, the use case for it, why we > need to store it in VFS objects, what threat model it is protecting > the system against, etc. [root@quaco ~]# file /bin/bash /bin/bash: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=160df51238a38ca27d03290f3ad5f7df75560ae0, for GNU/Linux 3.2.0, stripped [root@quaco ~]# file /lib64/libc.so.6 /lib64/libc.so.6: ELF 64-bit LSB shared object, x86-64, version 1 (GNU/Linux), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=8257ee907646e9b057197533d1e4ac8ede7a9c5c, for GNU/Linux 3.2.0, not stripped [root@quaco ~]# Those BuildID[sha1]= bits, that is present in all binaries I think in all distros for quite a while. This page, from when this was initially designed, has a discussion about it, why it is needed, etc: https://fedoraproject.org/wiki/RolandMcGrath/BuildID 'perf record' will receive MMAP records, initially without build-ids, now we have one that has, but collecting it when the mmap is executed (and thus a PERF_RECORD_MMAP* record is emitted) may not work, thus this work from Jiri. - Arnaldo > > > > However.. ;-) while using inode as build id storage place saves some memory > > by keeping just one copy of the build id for all file instances, there seems > > to be another problem. > Yes, the problem being that we can cache hundreds of millions of > inodes in memory, and only a very small subset of them are going to > have open files associated with them. And an even smaller subset are > going to be mmapped. > So, in reality, this proposal won't save any memory at all - it > costs memory for every inode that is not currently being used as > a mmapped elf executable, right? > > > The problem is that we read the build id when the file is mmap-ed. > > Why? I'm completely clueless as to what this thing does or how it's > used.... > > > Which is fine for our use case, > > Which is? > > -Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx
