On 2021-08-05 at 09:04, Thomas Schmitt wrote: > Hi, > >> Can someone tell me where I should look for the kernel for the >> loopback setting? > > Quite exactly a year ago i learned the hard way that it's > CONFIG_BLK_DEV_LOOP which on amd64 should be set to "m" to get /dev/loop*. > See its description at > https://sources.debian.org/src/linux/5.10.46-1/drivers/block/Kconfig/#L174 > > But i'm somewhat puzzled by this statement at the end > "Most users will answer N here."
That does seem odd; at least in the modern Linux world, loopback-device support is going to be expected if not required on the large majority of systems. I can't rule out that this description may have been accurate at one point, but unless an N here will mean that loopback-device support will be provided by some other code path, it does indeed not seem likely that it is what most users will/should say. It might be worth filing a documentation-level kernel bug report about this, or at least posting to the LKML to ask what the reasoning here is. > A code search yields no BLK_DEV_LOOP=n > > https://codesearch.debian.net/search?q=package%3Alinux+BLK_DEV_LOOP%3Dn&literal=0 > but lots of "y" and "m". > > (Wasn't there a way to inquire the running kernel's configuration ?) AFAIK, that's /proc/config.gz; it's present only if a specific Kconfig setting is enabled, and Debian stopped enabling that setting quite some years ago, apparently on the grounds that keeping the kernel config in memory at all times is unnecessarily wasteful (especially with the limited RAM that you see on e.g. embedded-type systems) vs. keeping it on-disk. At no later than that same time, Debian started putting the kernel config on-disk instead, under the filename /boot/config-`uname -r`. If - as the OP stated - the kernel config is not present in that location, then either there's information we're not being provided, or something is wrong. -- The Wanderer The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man. -- George Bernard Shaw
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