libshortrecv

Many special stream-oriented file descriptors in UNIX are expected to be allowed to read or write fewer bytes than requested by application. Primary example can be a TCP socket. One send can turn into multiple recvs or multiple sends can turn into one recv at the other end. In practive however when testing simple cases (e.g issueing small sends to 127.0.0.1) system may avoid short reads and writes, masking bugs in user programs.

This LD_PRELOAD library allows artificially decreasing buffer size parameter, simulating short reads or writes. Well-written programs are expected to stay as is, latently buggy programs can start to actually misbehave.

$ cmake .
$ make
$ LD_PRELOAD=$PWD/libshortrecv.so strace -e read,write cat
...
> ABCDEFGHIJKLMNOP
read(0, "ABCDEFGHIJKLMNOP\n", 79760)    = 17
write(1, "ABCDEFGH", 8)                 = 8
write(1, "IJ", 2)                       = 2
write(1, "KLMNOP", 6)                   = 6
write(1, "\n", 1)                       = 1
< ABCDEFGHIJKLMNOP

> kkkkkk
read(0, "kkkkk", 5)                     = 5
write(1, "kk", 2)                       = 2
write(1, "kk", 2)                       = 2
write(1, "k", 1)                        = 1
read(0, "k\n", 23675)                   = 2
write(1, "k", 1)                        = 1
write(1, "\n", 1)                       = 1
< kkkkkk

Features

• Intercepting functions
  • read
  • write
  • send
  • recv
  • sendto
  • recvfrom
  • readv
  • writev
• Environment variable configuration
  • LIBSHORTRECV_SEED - set random number seed. Use value 0 to force 1-byte reads or writes every time.
  • LIBSHORTRECV_NOREAD - disable intercepting of reading functions
  • LIBSHORTRECV_NOWRITE - disable intercepting of writing functions
  • LIBSHORTRECV_NOV - disable intercepting of readv/writev (which is untested at all)

Issues/Limitations

• Almost untested (LD_PRELOAD trick seems to fail to work with the program I indended to test this with, so I sort of abandoned the project)
• Not everything is interpeptable with LD_PRELOAD. strace-based or other closer-to-kernel solution would likely to work more reliably.
• Probably GNU/Linux-only.
• readv/writev interception code temporarily mutates iov which is supposed to be const.