Masahiro Yamada <masahiroy@xxxxxxxxxx> writes: > On Sat, Sep 7, 2024 at 10:26 PM Sam James <sam@xxxxxxxxxx> wrote: >> >> Sam James <sam@xxxxxxxxxx> writes: >> >> > Masahiro Yamada <masahiroy@xxxxxxxxxx> writes: >> > >> >> On Sat, Sep 7, 2024 at 12:29 AM Sam James <sam@xxxxxxxxxx> wrote: >> > >> > Hi Masahiro, >> > >> >>> >> >>> 50% or so of kernel builds within our package manager fail for me with >> >>> 'fixdep: read: success' because read(), for some reason - possibly ptrace, >> >>> only read a short amount, not the full size. >> >>> >> >>> Unfortunately, this didn't trigger a -Wunused-result warning because >> >>> we _are_ checking the return value, but with a bad comparison (it's completely >> >>> fine for read() to not read the whole file in one gulp). >> >>> >> >>> Fixes: 01b5cbe7012fb1eeffc5c143865569835bcd405e >> >> >> >> >> >> Fixes: 01b5cbe7012f ("fixdep: use malloc() and read() to load dep_file >> >> to buffer") >> >> >> > >> > Ah, thanks. I'll fix that and send v2 depending on how we decide to move >> > forward wrt below. >> > >> >> >> >> I guess, another approach would be to use fread() instead of read(). >> >> >> >> Does the attached diff fix the issue too? >> >> >> >> >> > >> > Unfortunately no. It failed for me in the same way as before :( >> > >> > The man page mentions: >> >> On success, fread() and fwrite() return the number of items read or >> >> written. This number equals the number of bytes transferred only when size is 1. >> > >> > so I guess it suffers from the same pitfall. I checked POSIX & ISO C as well >> > which says: >> >> If a partial element is read, its value is unspecified. >> > and >> >> The fread() function shall return the number of elements successfully >> >> read, which shall be less than nitems only if an error or end-of-file >> >> is encountered, or size is zero. >> > >> > The error reference is kind of mysterious there though. >> > >> > It kind of looks like fread *should* work. I'll send this mail and then >> > think about it a bit later and ask around to see if I'm missing >> > something obvious? >> >> OK, others disagree with my reading of fread and think it is ambiguous. >> >> With your patch, I was able to get failures albeit possibly less >> frequently. I'm trying my patch again in a loop now. >> >> > >> >> [...] >> > >> > thanks, >> > sam >> > > > > > > > Your quotation of the POSIX fread() spec: > (https://pubs.opengroup.org/onlinepubs/000095399/functions/fread.html) > >> If a partial element is read, its value is unspecified. > and >> The fread() function shall return the number of elements successfully >> read, which shall be less than nitems only if an error or end-of-file >> is encountered, or size is zero. > > > I think this is clear enough. > > > The end-of-file should not be encountered, as we check the file > size in advance. I believe that it's referring to the number of records, not if you read *1* record of size N. I looked at the musl and glibc sources and neither seem to retry partial reads in that case. I don't see any error indicator set.